One who is not a Christian or who opposes Christianity. (Merriam – Webster)
An unbeliever with respect to a particular religion. (Merriam – Webster)
One who acknowledges no religious belief. (Merriam – Webster)
If one person refers to another as an infidel, the first person is hostile towards the second person because that person has a different religion or has no religion. (Collins Dictionary)
(used especially between Christians and Muslims) someone who does not have the same religious beliefs as the person speaking: (Cambridge Dictionary)
A person who has no religion or whose religion is not that of the majority. (Oxford Dictionaries)
Infidel grunt style.