When Rome sacked Jerusalem in AD 70 and began imposing Fiscus Judaicus (Jewish Tax) across the empire, Gentile Christians were keeping Sabbath, the biblical feasts, and eating clean along with their Jewish brothers, so they were identified as Jewish by Rome and required to pay. Needless to say, many began looking for ways to distance themselves from anything that looked Jewish.
False teaching quickly emerged redefining the faith, decoupling it from its foundation in Torah, presenting another Jesus that had supposedly abolished the Law and started an entirely new religion. The constant monetary pressure ensured its eventual success in supplanting the original expression of our faith.
Rome ultimately adopted this new religion and its anti-Torah leadership, birthing Roman Catholicism, and eventually its protestant counterparts, which all continue to preach this other Jesus (2Co 11:4), one the early Church knew nothing about.
Christ Himself predicted this would happen (Mt 24:24); it makes perfect sense that the enemy would counterfeit God’s Messiah, making his impressions as life-like and attractive as possible. How can we tell the false from the true?
I think this is straightforward; simply go back to where the enemy started: restore the foundation — Torah. Identify any representation of Christ abolishing or minimizing the value and centrality of Torah as a false one, a counterfeit. God’s Son didn’t do this, and denounced anyone who did. (Mt 5:17-19)
This is not to say all who are caught up in false religion are necessarily separated from God; many are deceived and blinded by false teaching, believing and obeying what they do know as well as they can, given the circumstances. God will sort it all out, for sure.
We ought not to disdain those who are in error, for we ourselves also were sometimes foolish, disobedient, deceived. (Tit 3:3a) We should have compassion on the ignorant, and on them that are out of the way, seeing we ourselves also are compassed with infirmity. (He 5:2)
In meekness, we should instruct those that oppose themselves, hoping God will give them repentance to the acknowledging of the truth, that they may recover themselves out of the snare of the devil, who are taken captive by him at his will. (2Ti 2:25-26)