One of the classical examples of a man who was unquestionably sincere, and yet lost, out of Christ, and even an arch enemy of the early church, is that of Saul of Tarsus. Saul had even participated in the murder of the first Christian martyr, Stephen (Acts 7:58). He “....made havoc of the church, entering every house, and dragging off men and women, committing them to prison” (Acts 8:3). It is even reported further that “Saul, still breathing threats and murder against the disciples of the Lord, went to the high priest and asked letters from him to the synagogues of Damascus, so that if he found any who were of the Way, whether men or women, he might bring them bound to Jerusalem” (Acts 9:1-2). So here is a man who is religious about persecuting the way of Christ, believing Jesus to be an imposter. Yet, though misguided, he said, “....I have lived in all good conscience before God until this day” (Acts 23:1). He had been sincere, but that sincerity wasn't enough, because you see, sin is not just a matter of violating your conscience; it is a matter of violating the will of God. John was speaking of this in 1 John, Chapter 3, verse 4, when he said, “Whoever commits sin also commits lawlessness, and sin is lawlessness.”
In the course of time a Christian named Ananias informed Saul, “And now why are you waiting? Arise and be baptized, and wash away your sins, calling on the name of the Lord’” (Acts 22:16). Yet Saul’s story ends happily because when he learned the error of his way, he was sincere enough to turn from his sins to Christ and he went on to become the great Apostle Paul.
In Acts Chapter 2, we read that there were dwelling at Jerusalem, Jews, devout men, out of every nation under heaven (verse 5). Yet, as Peter preached to these sincere people, he accused them with these words: “Him, being delivered by the determined purpose and foreknowledge of God, you have taken by lawless hands, have crucified, and put to death” (Acts 2:23). Although they were devout, they had participated in the crucifixion of the Lord. On learning of their sin, they were (as the Bible says) "....cut to the heart, and said to Peter and the rest of the apostles, Men and brethren, what shall we do?” (Acts 2:37). They now realized that they were guilty before the Lord God. And as they were searching for a cleansing from this guilt, Peter said to them, “Repent, and let every one of you be baptized in the name of Jesus Christ for the remission of sins; and you shall receive the gift of the Holy Spirit” (Acts 2:38).
The role of sincerity is to enable a person to be fair and honest with the truth! Sincerity alone does not determine truth, scientifically, and medically, legally, or religiously. We can see then, what Jesus was saying when He said, “Not everyone who says to Me, 'Lord, Lord,' shall enter the kingdom of heaven, but he who does the will of My Father in heaven” (Matthew 7:21).