Don't want to restart an argument here wink but the term Infidel, meaning unfaithful (from latin) does tend to be associated with religion, and was particularly used as a slur on Muslims by Christians. 'The Infidel hoarde', etc, because it also means un-faithful, as in, they have no faith [in a Christian God].
I always took Vito's reference to Sollozzo as an 'Infidel' to mean that he considered him to possess lower values than Vito and his peers, because he had so embraced non-Christian Turkish culture. This coming from a traditionalist catholic I mean. For instance, technically speaking Sonny also practiced infidelity towards Sandra, but Vito didn't call him an Infidel, he just told him off for "all that comedy you're playing with that young girl" lol

Senator, we are both part of the same hypocrisy