The belief is popular: As long as a person is sincere in his devotion to God, God will save him. But the popularity of a belief does not guarantee that it is correct.
Consider the case of Paul. Prior to his becoming a Christian, he sincerely believed that he was rendering service to God by persecuting Christians. Consider his own testimony:
Indeed, I myself thought I must do many things contrary to the name of Jesus of Nazareth. This I also did in Jerusalem, and many of the saints I shut up in prison, having received authority from the chief priests; and when they were put to death, I cast my vote against them. And I punished them often in every synagogue and compelled them to blaspheme; and being exceedingly enraged against them, I persecuted them even to foreign cities (Acts 26:9-11).
On another occasion, Paul proclaimed, “I have lived in all good conscience before God until this day” (Acts 23:1). Later in life, the apostle described himself as “the chief of sinners” (1 Tim. 1:15). So even though Paul had been sincere in his service to God, he discovered he had been wrong and stood condemned—sincerity alone was insufficient!
While it is true that God requires man to render service from a genuine heart, obedience to His specific instructions—as revealed in the Bible—cannot be neglected or altered (cf. Rom. 6:17; Matt. 7:21-23).