Will converted civilizations actively spread your religion in Civ 6?

Question

Let's say I found a religion early and convert Spain (Phillip II). Will Phillip remain a religious fanatic, but spread my religion instead?

His description says he "Likes civilizations who follow the same Religion as him, and wants his cities to all follow the same Religion".

Can I expect that, at the very least, he will use missionaries, apostles and inquisitors (which have a special trait from Phillip: "eliminate 100% presence of other Religions") to keep his own cities following my religion?

That would be neat. But if it's not the case, I'd like to know, perhaps I should keep a closer eye on the civs I've already converted.

Answer 1

Yes and no.

When your cities get taken over, and you then spawn a missionary, you'll see that it's a missionary for the dominant religion in the city that spawned it.

The same if true for the AI when you convert their cities. Religious pressure will emanate from the AI's cities and it will spread.

However, it makes no sense that the AI would keep spreading the foreign religion, as they would then be helping you winning your religious victory which causes them to lose.

Yes, religion will emanate. No, the AI won't help you win the religious victory.

Answer 2

Mvemba a Nzinga will actively spread foreign Religion to his own cities using charges from the Apostles he gets for free when completing certain buildings on Religious cities.

He will also, with the same Apostles, attack passing units of other Religions, occasionally killing them and converting nearby cities with the area effect of lethal Theological Combat, regardless of who owns the cities.

Other than Mvemba, I don't think I ever saw a single Religious unit of a Religion not founded by the unit's owner. I have played for hundreds of hours over several difficulties. I nearly always go for Religious Victories, so I think I would have noticed.