C. M. Weimer already treated the grammatical issues. However, in light of the fact that this question is tagged classical-latin, and that the author is specifically not looking for a purely religious term, I believe it bears pointing out that peccator does not appear to be a classically attested word. It was apparently used by Lactantius (around 250–320) and Tertullian (around 150–220), excellent Latin writers, but not classical and decidedly Christian. And it is of course all over the Vulgate.
A classically attested form would be peccans (plural: peccantes). For example, you have Nepos (Agesilaus 5):
Cum [⋯] ut Corinthum oppugnaret multi hortarentur, negavit id suae virtuti convenire: se enim eum esse dixit, qui ad officium peccantis redire cogeret, non qui urbes nobilissimas expugnaret Graeciae.
When many urged him to attack Corinth, he said that this would not befit his chivalry; he was one, he said, to usher the sinners back on the right path, not to conquer the most famous cities of Greece.
While surely not Christian at all, you can see that the idea of sinners as “people who have strayed from the path of virtue” is present. For a stronger expression of moral outrage, you might want to consider the term nefarius (from nefas, the universal Roman term for all which goes against the laws of gods, men and nature), plural nefarii.