Note : every possible plot spoiler included here... but I don't care. Let's go. 
 So let me get this right. This miserable sourpuss atheistic type author guy Maurice meets this hot slutty (their word) woman Sarah who is married to England's most boring civil servant Henry. They have a full on steamy affair right under Henry's nose for four years and are very happy, except Maurice gives the impression that even when he's happy he's miserable. Like Morrissey. Similar name. Anyway, it's World War 2 and there's a big air raid and the guy goes downstairs to check on the cat or something and WHAM a great big German missile hits the building and a door falls on him. Sarah runs downstairs and sees his arm sticking out and thinks he's dead. She runs back upstairs and prays to God. So far so reasonable. But actually she doesn't believe in God. Well, people do strange stuff when they think their true love is dead under a door. She says if God makes Maurice not be dead then she will a) believe in God and b) give up Maurice. So when he wanders into her room all covered with dust and saying wow I just got hit by a door, I thought I was a goner but I just got a headache, how about that, she immediately thinks that God did it. She instantly takes it for a Miracle and not just a near-miss. And that's the last she sees of him, she cuts him off without a word of explanation, thus plunging them both into suicidal despair. (What she says is "Love doesn't end just because we don't see each other." Well, maybe, but shagging surely does.) 
 (Sample quote from Maurice after Sarah dumps him : 
 I thought : hating Sarah is only loving Sarah and hating myself is only loving myself. 
 Yes, it's psychobabble 20 years before the term was invented. ) 
 I suppose Sarah thinks that if she breaks the promise to God not to see him, then God will smite him completely dead with another door or handy piece of furniture. But this is not explained. 
 But hey, this was a promise made under duress. And anyway, if she just takes a moment to think, she will surely realise that during any air raid on London in the War, and any air raid anywhere at any time, many people will have prayed to the God they actually believed in that their loved ones would not die and many people will then have found their loved ones had died in horror and agony nevertheless. And some like her would find their loved ones had survived. So where's the logic in that? Well, there isn't any. It's just human nature. Unless we can conjecture that God sits there saying oh, that's a good prayer, very well expressed, very sincere, I'll answer that one. But those prayers are rubbish, cliche ridden, boring, really very bad, so those loved ones will have to fry. 
 So I figured that this dame was not at the front of the queue when they were handing out brains. 
 Actually, all these reasonable points are made by a Rational Atheist character who she goes to see to try to get him to argue her out of this insane piece of magical thinking. Oh the vow, the vow to the non-existent God! But it doesn't work : "his fanaticism fixed the superstition deeper". By the way, the Rational Atheist has a big Facial Disfigurement, which has Blighted his Life. I think this is some kind of symbol. 
 Then we get Sarah's diary and the full horror of her mind is laid bare. She loves Maurice, no, actually, she hates him. No, she loves him. But she hates God. No, she doesn't believe in him. Oh wait, she loves God, who she doesn't believe in. And she thinks she's a Catholic (no other varieties are available in Graham Greene's universe - Methodism or Zoroastrianism don't get a look in). She thinks she might be a Catholic but she doesn't believe! Hold the phone, yes she does. God! No God! Oh the pain! The pain! Love, hate, hate, love, belief, maybe - wah wah wah. Wah Wah. 
 It becomes really tiresome. 
 Sample diary quote: 
 How good You (=God) are. You might have killed us with happiness, but You let us be with You in pain.... Dear God, you know I want to want Your pain... 
 So eventually this majorly troubled woman ends up saying "I've caught belief like a disease" and then dies. 
 I see that many people think The End of the Affair is a very beautiful meditation on love and faith but it seemed to me as if it was presenting religious faith as if it's something lying in wait to trap the mentally exhausted person at their lowest point. Really quite nasty. If that was the end of The End of the Affair I could have been okay with this novel, and tried to overlook the tedious love-hate-pain-God loop tape playing throughout, but uh-oh, Greene gives his tortured tale a totally tendentious twist right at the end, where the Rational Atheist is CURED of his facial disfigurement because he snipped off a lock of the hair of the dead Sarah and slept on it. Now really, Mr Greene, pull the other one. It's got bells on. 
 Conclusion: a mixed-up mess that doesn't work on any level which inexplicably gets included in 100 Best Novels lists, proving, once again, yes, I'm on the wrong planet.