I’m categorically not touching any of the religious discussion anything in this thread. No way no how.
I would be worried for your friend. One, you describe a very very large swing in a very very short time. Suicidal to euphoric is a huge range. People don’t usually do that pretty much overnight. He sounds like he’s so preoccupied with the new feeling that he can’t recall what it was like to feel the other way, which is a bit of a bad sign – if nothing else, those who cannot remember history are generally doomed to repeat it.
Two, his ability to explain what he’s thinking and his expectations that you will understand are badly mismatched. You say he expected you to get what he got after a brief surface explanation, when what he’s trying to get across is clearly something he feels goes very deep. It suggests he doesn’t really know how he sounds outside of his own head, or can’t distinguish in short-term memory between things he’s said aloud and things he’s cueing up to say that don’t manage to escape.
Three, he seems to be persisting in behaviors that make you uncomfortable. If he’s talking to you about his new feelings, you’re probably someone he should know well enough to know what level of penis-chat you will and will not be comfortable with. This suggests that either he isn’t able to self-monitor, or he isn’t able to keep long-term knowledge about you in the forefront of his mind.
And four, citalopram shouldn’t be discontinued suddenly. It has a rather short half-life, and brain transmitters do not re-normalize themselves fast enough to compensate. If nothing else, he may be at risk for something like seizures, particularly if he’s adding other drugs to the mix. You may wish to put in a call to whoever gave him the citalopram, if you can find the information. It might be on the pill bottle. The doctor can’t tell you diddly about your friend, but there’s no law that says you can’t leave a message for the doctor describing what’s going on.
Most people, whether they’re having a revelation or a manic episode, are not actually out to harm others. People who aren’t thinking clearly, however, may perceive threats where none exist, or fail to recognize the risk in what they’re doing. A much more likely danger is that you’re going to get stuck doing all the housework and possibly repairing things for the forseeable future, as he’s going to be much too involved in whatever he’s got going on to take care of things on Planet Earth. If this is mania, you may also get awakened at three in the morning when he needs someone to talk to. Good luck.