Refers to a follower of a specific religion; often used as a suffix in compounds like キリスト教徒 (Christian) or 仏教徒 (Buddhist).
He is a devout Christian.
There are many Buddhists in Japan.
There are many adherents of Islam all over the world.
He became a Buddhist upon getting married.
As a believer, he prays every day without fail.
信徒 is a slightly more formal, often church-related term for a religious follower, largely interchangeable with 教徒 but common in Christian contexts.
異教徒 means a believer of a different (often opposing) religion, whereas 教徒 is the neutral term for a believer of any religion.
Compound of 教 (きょう, 'teaching, religion') and 徒 (と, 'follower, party').