From 1 Timothy Chapter 1:
12 I thank Christ Jesus our Lord, who has strengthened me, because He considered me faithful, putting me into service,13even though I was formerly a blasphemer and a persecutor and a violent aggressor. Yet I was shown mercy because I acted ignorantly in unbelief;14and the grace of our Lord was more than abundant, with the faith and love which are found in Christ Jesus.
In the letter Paul writes to Timothy, he refers to the Law as being for “those who are lawless and rebellious”. He also states that it is good if it is used lawfully. Despite these statements, Paul, who was a student of the Law, remained unaware of his own desperately sinful behavior until the Resurrected Christ appeared before him.
Paul had a fantastic pedigree. He was a great student. He was applying what He learned to the point of persecuting what He perceived as Heresy which came out of Judaism.
The Law never led Paul to recognizing his need for a savior. In fact, it would appear that his learning had the opposite effect. It hardened him toward the Gospel. By his own admission, he was a “blasphemer and a persecutor and a violent aggressor”, but he didn’t recognize it at the time.
Perhaps Saul believed that he was serving God by being zealous for the true faith. A faith founded on God’s utter holiness and demands that Israel be holy, too. By purging the evil of Christianity, he was helping to do the “work of God”. This include the killing of Christians.
You could have addressed Paul with the Law, but it wouldn’t have done any good. He would have run circles around you. In the case of Paul, it took the direct revelation of the risen Christ to shut down all references to his righteousness, genealogy, or his zeal.
Like Yoda talking to Luke Skywalker, Paul had to “unlearn”(the righteousness through law-keeping) and had to learn Christ and Him crucified for the forgiveness of sins.
It all started with the revelation of Jesus Christ. We can’t expect this for every religious anti-theist that comes along. But we can look to the revelation of Jesus Christ in His word, especially the Gospel (found in Matthew, Mark, Luke and John). And we can proclaim Christ and Him crucified. These are the means that God has given us to set free the captives … those captive to Law-keeping.