I found the answer to my own question. An infidel is any non-baptized person. From NewAdvent:
As in ecclesiastical language those who by baptism have received faith in Jesus Christ and have pledged Him their fidelity and called the faithful, so the name infidel is given to those who have not been baptized. The **term applies ** not only to all who are ignorant of the true God, such as pagans of various kinds, but also to those who adore Him but do not recognize Jesus Christ, as Jews, Mohammedans; strictly speaking it may be used of catechumens also, though in early ages they were called Christians; for it is only through baptism that one can enter into the ranks of the faithful. Those however who have been baptized but do not belong to the Catholic Church, heretics and schismatics of divers confessions are not called infidels but non-Catholics.
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