Do you mean that they, or even you, could really be closet Moslems and there is no test for it, nothing I could ask that could detect it ? It seems to me fascinating that you suggest this. No one would be able to distinguish a Moslem street preacher from a Protestant/Fundamentalist/Evangelical/Islamic street preacher ...
Uh, yeah a muslim could lie to anyone they want. And if you are a “casual” or “baby” Christian they make take you in. I have no idea where you come with the comment that “there is no test for it, nothing I could ask that could detect it” Much of the NT is spend stressing the importance of doctrine and being on guard for phony Christians. That’s why it’s important to be steeped in scripture and genuine Christian doctrine. Anyone who would disrupt and harass a Christian service is pretty obviously not a follower of Jesus Christ. These are just muslim infiltrators. The extreme left wing antis who disrupt services don’t pretend to be Christians themselves.