Is Sincerity Enough?

Nope! And the case of Paul the apostle confirms this answer. Prior to his becoming a follower of Jesus, Paul sincerely believed he was obeying 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). He also described himself as “the chief of sinners” (1 Tim. 1:15).

Even though the apostle had lived his former life in genuine sincerity, honestly believing himself to be a servant of God, he was wrong. While God requires man to render service from a genuine heart (cf. Rom. 6:17), He also obligates him to render obedience to His specific instructions (cf. Matt. 7:21-23).