Enter Plato and Aristotle. They agree with Parminides that everything around us must necessarily exist, but in their observations of the universe at the same time they say that not everything is a concrete reality. As a whole, the world must necessarily exist and is eternal, but when we look at the world in terms of its individual parts, these things may or may not exist. Individual things are therefore contingent as opposed to having necessary existence.
Now this is baffling to them as they try to philosophize the nature of the universe. While they have adopted the Aristotelian concept of the universe that God must necessarily exist and that all other created beings are contingent, that is, may or may not exist according to His Will, they also insist in holding on to the Parminidean concept of the universe.
They then attempt to reconcile these two contradictory concepts. If the things around us; the trees, the animals, matter and mass, may or may not exist at one instance, why is that we are able to observe them, study them, and make some conclusions about them? Something that can be observed must exist and have some form of concrete existence to be studied and understood right? Maybe physically, all those things may or may not exist, but in the mind it does have some form of concrete existence which is why we are able to understand them?
Enter Thomas Aquinas, one of the greatest Christian theologian and metaphysician of the Medieval era. Aquinas was thinking, if all things must exist and at the same time may or may not exist, this may mean that there is a difference in between what is conceived and thought of in the mind and what actually exists outside the mind. Thus, Aquinas concluded that there must be a difference between what is conceived in the mind (essence) and what actually is outside the mind (existence); a real difference between essence and existence. He elucidated this point by bringing up the pink unicorn example. One can think of a pink unicorn in the mind, and then describe the characteristics of the pink unicorn. A pink unicorn is an animal that looks like a horse, is pink in colour and has a horn on its head. But a pink unicorn does not exist outside the mind! Therefore, to him, there is a difference between what is in the mind, and what is outside it. A thing that has only essence has the potential to exist, but this existence is not actual. If God created the world according to his Will, then the world's essence must have come before its existence. Meaning to say, the universe was in God's thoughts before it was manifested into reality. But God has no beginning, therefore God's own essence cannot precede His existence. God, therefore, is pure existence, whose essence is also existence.
Enter Ockham. Arguing from the same premises that there is a difference between essence and existence, and that things can be understood as essences but may not actually exist, then no amount of knowledge of the world can possibly tell us whether something exists or not. Since our knowledge and understanding of the world is based on our observation of things that are contingent; things that we cannot be sure whether it exists or not, then objectivity of things cannot be established, including the idea of God. How can one be completely sure that God exists then if our understanding of things are merely based on things whose existence we doubt?
This is what is termed by them as 'the problem of God,' which is rather amusing. God has no problems, people do!
Enter Descartes. Descartes argued in the same mould as Ockham; we cannot determine with certainty that anything actually exists outside the mind. But at the same time, Descartes also said that since we know ourselves as an observer, then the only certainty that we are able to establish is the fact that the observers or "I" exist. This far-reaching conclusion would then be officially termed as the Cartesian doubt. Descartes, being a devout Christian, then attempted to argue that since "I" exists, God must necessarily exist as well. But the problem is, while Descartes was able to demonstrate by means of empirical intuition that the self exists, he was unable to use the same methods to prove God's existence. The problem becomes more complex because God's Essence cannot be known, and because earlier they have established that God's Essence and Existence are the same, therefore God's Existence cannot be known as well.
Enter Kant. Kant asserts that since His Existence cannot be demonstrated rationally, a feat that is impossible to achieve, therefore the Existence of God is just a matter of faith. Like the story of the Pink Unicorn, God is just a concept that resides in the mind.
Which is why Christianity's notion of faith is significantly different from the Islamic concept of Iman. The belief in God in the Christian sense is more of a conjecture or a doubt; "yes I believe in God, but at the same time I do not reject the possibility that God may not exist" or "yes I believe in God, but I am in the middle right now. While I am sure that God exists, I am also equally sure that God does not exist." This is the conclusion of reason unguided by Revelation!
Therefore because God is just a concept in the mind and not a Reality, the concept becomes relative, subject to different interpretations. A more contemporary interpretation of the concept of God would be based on Husserl's ideas on phenomenology. Husserl extends the arguments forwarded by Descartes, saying that because the world has an uncertain existence, existence is an insoluble problem, including the existence of God. Hence, he says that we should not be bogged down by these problems that will continue to be unresolved. Because we know with certainty that we exist, therefore we can also be certain that the things around us exist as objects of consciousness for us. We may not be able to prove that these things exist, but we can know for certain that we are conscious of them. This means that we should instead focus on our own consciousness and experiences; these are the things that we can be certain about. Our own consciousness and experience then becomes the new God.
As Heidegger puts it "We are ourselves the entities to be analysed."
Enter Heidegger who, studying the works of Husserl, forms his own conclusion with regards to existence and certainty. In our consciousness, things fold and unfold in the passage of time. It is time that created our consciousness of things. Therefore in his book "Being and Time", he concludes that "Being is Time." Therefore, Time is the new God.
Therefore it no surprise that because Time is God in contemporary Christian theology, they always argue for change, development and progress. Since Time always flow forward, always changing, so does religion and its concepts and values. Religion must follow Time. Beliefs must be open-ended in line with historical development standards, they say.
This is how religion is popularly conceived today as a result of the amalgam and development of these ideas. Unfortunately, this is how Islam is conceived among those lacking in understanding. Without being deeply rooted to the fundamental elements of our worldview, our Ru'yat al-Islam Lil Wujud, it is very possible that we might be swept astray.
May Allah show us that the truth is true, and that falsehood is false.