there must be 3 types of conversion.
Religious. By a priest not all vive are missionaries. But there are cults that capture the imagination.
Those who come from the Middle East and India have a lot to say. Mitraism, Mystical philosophies, cult to Isis, some times Judaism, Buddhism.
Bribery. The other is loyalty by money.
It depends on loyalty.
Submission. That of an animal or a slave.
Instant capture if the master is not there.