user533: If everything is impermanent and this is used as a reason why Buddhists do not believe in a creator and eternal God, how can Nirvana be permanent?
Did the Buddha ever explain this point? 

user254: Let me try a simple answer.
"Everything" is impermanent because everything is a "compound thing" which has no "independent existence". For example, here we are on a planet: sometime in the astronomical future there will no longer be a planet (because of the lifecycle of our solar system's sun) therefore "things" which depend on the existence of the planet will be not continue to exist: these things are impermanent.
Similarly each human body, each mountain, each city, each relationship is a "compound thing" which is not permanent.
But Nirvana is not a "compound thing". It can be described as an ending which does not begin again.

I think it's worth reading this description of Nirvana (if only because it goes a long way towards explaining the Fire sermon, which is famous).

Forgetting what I said above above the planet ending, there's another (perhaps non-Buddhist) tradition which says that "Nothing lasts forever but the earth and sky".
Seen in that light, I once saw a movie which was set in probably the lower hills of Nepal. A character is walking and comes to a kind of cairn that is a pile of face-down slates, on which people have written messages.
He picks one up in his hand and turns it over to read its message, which says,

How do you make a drop of water last forever?
Put it in the ocean.


The following may be like explaining a koan so the following is spoiler text:

 - It's by loosening the attachments it has to/within itself that the drop spreads through the ocean
 - Having dissolved its self that drop will never reappear (there's no such thing as the same drop)
 - The 'ocean' is also used for various metaphors: including for Dhamma, for Freedom, for Unbinding, for Heedfulness; but it's also used as a metaphor for forms, including forms of the mind i.e. Dhamma.


Beware that samsara might be endless? So that some attempts to describe the permanence of nirvana might be describing, instead, the endlessness of samsara?
Maybe samsara is not endless, though; it is perhaps beginningless (Assu Sutta and Timsa Sutta), however it has (or can have) an end; or at least dukka if not samsara can have an end: that is the Third Noble Truth.
There seems to be some debate about what causes (is at the root of) samsara.

Some say that it's the Three Poisons
Others suggest that ignorance is the root cause.

If you accept the view that it's caused by the three poisons, apparently you can gain "non-returning" by thoroughly abandoning any one them (except that, I don't know, some say that 'non-returning' requires you to abandon other 'fetters' too).
If you accept that it's caused by ignorance perhaps the antidote is knowing (knowing continuously? knowing endlessly?) the four noble truths.

user471: Nibbana is not a state of mind or state of being or state of consciousness. Perhaps, I can describe it as "the complete absence of suffering".
It is what the mind experiences when it is completely free from defilements, suffering and discontent. It is a mental object.
It is also the only unconditioned unborn phenomena. It's permanent and it's always there whether you experience it or not.
Also, please see this answer for details.

Ven: Sariputta: “Reverends, extinguishment (Nibbana) is bliss!
Ven. Udayi: “But Reverend Sāriputta, what’s blissful about it, since
nothing is felt?”
Ven. Sariputta: “The fact that nothing is felt is precisely what’s
blissful about it (Nibbana).”
AN 9.34


The born, come-to-be, produced,
The made, the conditioned, the transient,
Conjoined with decay and death,
A nest of disease, perishable,
Sprung from nutriment and craving's cord —
That is not fit to take delight in.
The escape from that, the peaceful,
Beyond reasoning, everlasting,
The not-born, the unproduced,
The sorrowless state that is void of stain,
The cessation of states linked to suffering,
The stilling of the conditioned — bliss.
Iti 43


user43: This very point is a subject of doctrinal disagreement between Mahayana and Theravada. According to Theravada, Nibbana has svabhava (self-nature) that is unconditioned, deathless and totally transcendent to the conditioned world. But according to Mahayana (specifically Madhyamika), the fact that Nirvana is called shunyata means it does not have svabhava, just like everything else. In other words, according to Mahayana, Nirvana is permanent because it does not exist, or to quote Chogyam Trungpa it is "completely indestructible because it is not existent".
As my current teacher says, "Nothing is permanent, everything changes -- except one thing. Which one thing? The fact that everything changes." This is a hint ;) What we call Nirvana is not something "transcendent to the conditioned world", it is the self-existing nature of everything.
According to Mahayana, Nirvana is the very nature of Samsara. The three marks of existence are non-escapable. Any attempts to run away from them are part of the wanderings in Samsara. Only when this escape is brought to an end, when we clearly see that our hope to reach Enlightenment was a hope to escape the three marks of existence, then we can finally loose the ground under our feet, fall through space, open our eyes and look around, and see unarisen Nirvana and self-existing Enlightenment. I hope it's clear now.

user18: There is a famous set of verses of the Buddha that go as follows:


“All conditioned things are impermanent” – when
  one sees this with wisdom, one turns away from
  suffering. This is the path to purification.
“All conditioned things are unsatisfactory” –
  when one sees this with wisdom, one turns away
  from suffering. This is the path to purification.
“All things are not-self” – when one sees this
  with wisdom, one turns away from suffering.
  This is the path to purification.

-- Dhp. 277-9 (Buddharakkhita, trans)

Do you see the difference?
Nowhere did the Buddha say that everything is impermanent, because it is not true. All things that arise must cease (SN 56.11), and that is the key to answering this question, because nibbaana is unarisen:

And what is the noble search? Here someone being himself subject to birth, having understood the danger in what is subject to birth, seeks the unborn supreme security from bondage, Nibbāna;
-- MN 26 (Bodhi, trans.)

The logical conclusion of not being born is not dying, and so:

being himself subject to death, having understood the danger in what is subject to death, he seeks the deathless supreme security from bondage, Nibbāna;
-- Ibid


533: <p>If everything is impermanent and this is used as a reason why Buddhists do not believe in a creator and eternal God, how can Nirvana be permanent?</p>

<p>Did the Buddha ever explain this point? </p>
 254: <p>Let me try a simple answer.</p>
<p>&quot;Everything&quot; is impermanent because everything is a &quot;compound thing&quot; which has no &quot;independent existence&quot;. For example, here we are on a planet: sometime in the astronomical future there will no longer be a planet (because of the lifecycle of our solar system's sun) therefore &quot;things&quot; which depend on the existence of the planet will be not continue to exist: these things are impermanent.</p>
<p>Similarly each human body, each mountain, each city, each relationship is a &quot;compound thing&quot; which is not permanent.</p>
<p>But Nirvana is not a &quot;compound thing&quot;. It can be described as <a href="http://www.accesstoinsight.org/tipitaka/kn/ud/ud.8.01.than.html" rel="nofollow noreferrer">an ending</a> which does not begin again.</p>
<hr />
<p>I think it's worth reading <a href="http://www.accesstoinsight.org/lib/authors/thanissaro/nibbana.html" rel="nofollow noreferrer">this description of Nirvana</a> (if only because it goes a long way towards explaining <a href="http://www.accesstoinsight.org/tipitaka/sn/sn35/sn35.028.nymo.html" rel="nofollow noreferrer">the Fire sermon</a>, which <a href="http://en.wikipedia.org/wiki/Fire_Sermon" rel="nofollow noreferrer">is famous</a>).</p>
<hr />
<p>Forgetting what I said above above the planet ending, there's another (perhaps non-Buddhist) tradition which says that &quot;Nothing lasts forever but the earth and sky&quot;.</p>
<p>Seen in that light, I once saw a movie which was set in probably the lower hills of Nepal. A character is walking and comes to a kind of <a href="http://en.wikipedia.org/wiki/Cairn#Asia_and_the_Pacific" rel="nofollow noreferrer">cairn</a> that is a pile of face-down slates, on which people have written messages.</p>
<p>He picks one up in his hand and turns it over to read its message, which says,</p>
<blockquote>
<p>How do you make a drop of water last forever?</p>
<p>Put it in the ocean.</p>
</blockquote>
<hr />
<p>The following may be like <a href="https://buddhism.meta.stackexchange.com/a/272/254">explaining a koan</a> so the following is <a href="https://buddhism.meta.stackexchange.com/a/277/254">spoiler text</a>:</p>
<blockquote class="spoiler">
<p> - It's by loosening the attachments it has to/within itself that the drop spreads through the ocean<br />
 - Having dissolved its self that drop will never reappear (there's <a href="https://skeptics.stackexchange.com/a/23138/2703">no such thing</a> as the same drop)<br />
 - The 'ocean' is also used for various metaphors: including for <a href="http://www.accesstoinsight.org/ptf/dhamma/" rel="nofollow noreferrer">Dhamma</a>, for <a href="http://www.accesstoinsight.org/lib/authors/bodhi/bl071.html" rel="nofollow noreferrer">Freedom</a>, for <a href="http://www.accesstoinsight.org/tipitaka/sn/sn35/sn35.200.than.html" rel="nofollow noreferrer">Unbinding</a>, for <a href="http://www.accesstoinsight.org/tipitaka/an/an10/an10.015.than.html" rel="nofollow noreferrer">Heedfulness</a>; but it's also used as a metaphor for <a href="http://www.buddha-vacana.org/sutta/samyutta/salayatana/sn35-229.html" rel="nofollow noreferrer">forms</a>, including forms of the mind i.e. Dhamma.</p>
</blockquote>
<hr />
<p>Beware that samsara might be endless? So that some attempts to describe the permanence of nirvana might be describing, instead, the endlessness of samsara?</p>
<p>Maybe samsara is not endless, though; it is perhaps beginningless (<a href="http://www.accesstoinsight.org/tipitaka/sn/sn15/sn15.003.than.html" rel="nofollow noreferrer">Assu Sutta</a> and <a href="http://www.accesstoinsight.org/tipitaka/sn/sn15/sn15.013.than.html" rel="nofollow noreferrer">Timsa Sutta</a>), however it has (or can have) an end; or at least dukka if not samsara can have an end: that is the <a href="http://www.accesstoinsight.org/ptf/dhamma/sacca/sacca3/" rel="nofollow noreferrer">Third Noble Truth</a>.</p>
<p>There seems to be some debate about what causes (is at the root of) samsara.</p>
<ul>
<li>Some say that it's the <a href="http://en.wikipedia.org/wiki/Three_poisons_(Buddhism)" rel="nofollow noreferrer">Three Poisons</a></li>
<li><a href="http://en.wikipedia.org/wiki/Bhavacakra#Within_the_Theravada_tradition" rel="nofollow noreferrer">Others suggest</a> that <a href="http://en.wikipedia.org/wiki/Avidy%C4%81_(Buddhism)" rel="nofollow noreferrer">ignorance</a> is the root cause.</li>
</ul>
<p>If you accept the view that it's caused by the three poisons, apparently you can gain &quot;non-returning&quot; by <a href="http://www.accesstoinsight.org/tipitaka/kn/iti/iti.1.001-027.than.html" rel="nofollow noreferrer">thoroughly abandoning any one them</a> (except that, I don't know, some say that '<a href="http://en.wikipedia.org/wiki/An%C4%81g%C4%81mi" rel="nofollow noreferrer">non-returning</a>' requires you to abandon <a href="http://en.wikipedia.org/wiki/Fetter_(Buddhism)" rel="nofollow noreferrer">other 'fetters'</a> too).</p>
<p>If you accept that it's caused by ignorance perhaps the antidote is knowing (knowing continuously? knowing endlessly?) the four noble truths.</p>
 471: <p>Nibbana is not a state of mind or state of being or state of consciousness. Perhaps, I can describe it as &quot;the complete absence of suffering&quot;.</p>
<p>It is what the mind experiences when it is completely free from defilements, suffering and discontent. It is a mental object.</p>
<p>It is also the only unconditioned unborn phenomena. It's permanent and it's always there whether you experience it or not.</p>
<p>Also, please see <a href="https://buddhism.stackexchange.com/a/31737/471">this answer</a> for details.</p>
<blockquote>
<p>Ven: Sariputta: “Reverends, extinguishment (Nibbana) is bliss!</p>
<p>Ven. Udayi: “But Reverend Sāriputta, what’s blissful about it, since
nothing is felt?”</p>
<p>Ven. Sariputta: “<strong>The fact that nothing is felt is precisely what’s
blissful about it (Nibbana).</strong>”<br />
<a href="https://suttacentral.net/an9.34/en/sujato" rel="nofollow noreferrer">AN 9.34</a></p>
</blockquote>
<blockquote>
<p>The born, come-to-be, produced,<br />
The made, the conditioned, the transient,<br />
Conjoined with decay and death,<br />
A nest of disease, perishable,<br />
Sprung from nutriment and craving's cord —<br />
That is not fit to take delight in.</p>
<p>The escape from that, the peaceful,<br />
Beyond reasoning, everlasting,<br />
The not-born, the unproduced,<br />
The sorrowless state that is void of stain,<br />
The cessation of states linked to suffering,<br />
<strong>The stilling of the conditioned — bliss.</strong><br />
<a href="https://www.accesstoinsight.org/tipitaka/kn/iti/iti.2.042-049x.irel.html#iti-043" rel="nofollow noreferrer">Iti 43</a></p>
</blockquote>
 43: <p>This very point is a subject of doctrinal disagreement between Mahayana and Theravada. According to Theravada, Nibbana has <em>svabhava</em> (self-nature) that is unconditioned, deathless and totally transcendent to the conditioned world. But according to Mahayana (specifically Madhyamika), the fact that Nirvana is called <em>shunyata</em> means it does not have <em>svabhava</em>, just like everything else. In other words, according to Mahayana, Nirvana is permanent because it does not exist, or to quote Chogyam Trungpa it is "completely indestructible because it is not existent".</p>

<p>As my current teacher says, "Nothing is permanent, everything changes -- except one thing. Which one thing? The fact that everything changes." This is a hint ;) What we call Nirvana is not something "transcendent to the conditioned world", it is the self-existing nature of everything.</p>

<p>According to Mahayana, Nirvana is the very nature of Samsara. The three marks of existence are non-escapable. Any attempts to run away from them are part of the wanderings in Samsara. Only when this escape is brought to an end, when we clearly see that our hope to reach Enlightenment was a hope to escape the three marks of existence, then we can finally loose the ground under our feet, fall through space, open our eyes and look around, and see unarisen Nirvana and self-existing Enlightenment. I hope it's clear now.</p>
 18: <p>There is a famous set of verses of the Buddha that go as follows:</p>

<blockquote>
  <ol start="277">
  <li><p>“<strong>All conditioned things</strong> are impermanent” – when
  <br/>one sees this with wisdom, one turns away from
  <br/>suffering. This is the path to purification.</p></li>
  <li><p>“<strong>All conditioned things</strong> are unsatisfactory” –
  <br/>when one sees this with wisdom, one turns away
  <br/>from suffering. This is the path to purification.</p></li>
  <li><p>“<strong>All things</strong> are not-self” – when one sees this
  <br/>with wisdom, one turns away from suffering.
  <br/>This is the path to purification.</p></li>
  </ol>
  
  <p><em>-- Dhp. 277-9 (Buddharakkhita, trans)</em></p>
</blockquote>

<p>Do you see the difference?</p>

<p>Nowhere did the Buddha say that everything is impermanent, because it is not true. All things that arise must cease (<a href="http://www.accesstoinsight.org/tipitaka/sn/sn56/sn56.011.nymo.html">SN 56.11</a>), and that is the key to answering this question, because <em>nibbaana</em> is unarisen:</p>

<blockquote>
  <p>And what is the noble search? Here someone being himself subject to birth, having understood the danger in what is subject to birth, seeks the unborn supreme security from bondage, Nibbāna;</p>
  
  <p><em>-- MN 26 (Bodhi, trans.)</em></p>
</blockquote>

<p>The logical conclusion of not being born is not dying, and so:</p>

<blockquote>
  <p>being himself subject to death, having understood the danger in what is subject to death, he seeks the deathless supreme security from bondage, Nibbāna;</p>
  
  <p><em>-- Ibid</em></p>
</blockquote>
