It’s not only right to criticize false religion, no matter whose religion it is, it’s the faithful Christian’s responsibility! Jesus criticized the Pharisees for “teaching as doctrines the commandments of men” (Matt 15:7-9). Paul criticized those who tried to bind the shadows of the Mosaic law on New Testament Christians (Col 2:16-23). Peter warned that false teachers would “secretly bring in destructive heresies” (2 Pet 2:1-3). Elders and preachers are to rebuke sin and false doctrine (2 Tim 4:1-5 f; Titus 1:10-14). Christians are to “contend earnestly for the faith” (Jude 3), while having no fellowship with sin and error (2 Cor 6:14-18).
Today many people have the mistaken idea that if someone believes in God and worships Him sincerely from the heart, it’s unkind, unloving, and unchristian to criticize his religious practices. However, false doctrine (Ephesians 4:14), ignorant worship (Acts 17:23), and vain religion (Jam 1:27) do exist, and all the sincerity, zeal, sacrifice, and devotion in the world will not make wrong things right (Acts 26:9-11). Remember how Elijah the prophet mocked the 450 false prophets of Jezebel and her false Baal religion (1 Kin 18). Many sincere, zealous, religious people are going to be very surprised on Judgment Day when they are rejected by the Lord (Matt 7:21-23).
Actually the sincere religious person who is unknowingly following the commandments of men is in greater spiritual danger than the irreligious reprobate in the gutter, because at least the reprobate knows that he’s lost. Keep in mind, false religions mock the messengers of God, but they don’t have truth on their side (2 Chron 36:16). To save those who are in religious error, someone must speak the truth (Jam 5:19,20). Obviously, he must speak the truth in love (Eph 4:15), but speak the truth he must, and no lie is of the truth (1 Jn 2:21).