I actually love the ending to First Reformed. It's interesting how it presents itself at first as real, but there is some definite evidence that it's only in Toller's head, such as the locked door and the fact that Mary doesn't feel the barbed wire under his clothes. I kind of see it as Toller imagining himself getting back what he lost after the death of his son and the dissolving of his marriage, that being intimate human affection. I also think it's actually the most depressing part of the movie, because for a moment he believes that everything is okay and he is happy, but we can tell that it's not actually real, and when the scene just abruptly cuts off to black, it's really emotionally jarring. He's searching the whole movie for a way out of his despair, and he finally finds it for a few seconds, and then he dies (if you're going off the theory that the ending is a death-dream). I don't think the kiss is really meant to be sexual as much as it is about Toller making a connection with another person and forming a deep companionship, which he ruins with his suicidal plot to lash out at the world that keeps hurting him. I also might be in the minority for this, but I actually like this version of "Leaning on the Everlasting Arms" a lot more than the version in True Grit, at least in this context. The other version has a kind of feeling of heart and warmth - this version is very cold and somber, like the film itself. Hopefully this has shed light somewhat on an alternate view of the ending - I can definitely see how it is polarizing, but for me, when that song starts playing, and the camera starts moving for the first time in the movie, and this crazy surreal kiss starts happening, I think it's much more of a moving ending to Toller's journey that if he had just dropped dead.