Infidel is a nasty way of referring to someone who does not follow the same religion you do. How can you call people infidels and expect them to believe your religion is tolerant?
Infidel comes to English from the Latin infidelis, which means “unfaithful,” although it’s almost always hurled at a person of different faith. You can also use the word more lightly to mean someone who doesn’t share a common belief or opinion. Cries of “Infidel!” could be heard throughout the dorm when you announced that you didn’t root for the local football team.