This is too good to immediately put into measured words, so allow me to get this out of my system:
First Reformed is stunning. Images of startling power are delivered so fiercely and confidently. While the whole film is a goldmine of technical and creative firepower, the third act had me genuinely shaken. It's a clichéd plaudit, but this truly is Hawke's performance of a lifetime. All his versatile experiences have led him to this point, and he is a master at work here.
This is jaw-dropping cinematic goodness. I'm not close to done ruminating on this. More to come.