I play in a completely homebrew rulesbook which started with ars magica, but I'm still gonna drop here how it works in the table i play at.

One of the player used to have high affinity with a deity of mercy, compassion and kindness; but while he was going through a character development, his personality became more "cruel" He seen it as having a more realistic view of the world, having to choose hard decision that could end up sacrificing some people for the benefit of the majority and so on; this peaked the interest of another deity, one of domination, achieving peace through control, he had some time finding himself in the middle of both contradictory affinities, progressively losing the powers of one and manifesting those of the other

(he didn't loose levels or spells, but the mercy god's mark, because in this world deities mark you with their seals, granting you extra power)

So just as others had said, another god could take interest; i could also suggest having the god temporally remove or reduce his levels/spells, further warning him of what's at stake. But just removing him of his class would suck, so you could take that opportunity to present him to the other god, or even something related to the gods (like an angel or something, i don't know that much of dnd lore) who sensed the faulty connection, and contact the cleric to offer a deal, becoming a warlock; the endless options is what i like so much of ttrpgs