I believe Saul of Tarsus to be the most striking example of conversion in the New Testament.
He clearly did not believe Jesus as the Christ. We are allowed, by inspiration, to look into Saul’s mind. The account says, “Saul still breathing threats and murder,” (Acts 9:1 ESV). Literally, Saul was threatening and murdering Christians with every breath he took. That shows his mind. He was 100 percent against Jesus.
But something happened to change that. That something was seeing the resurrected Christ on the road to Damascus. Seeing Christ alive after Saul knew he had been crucified made an indelible impression on him. The text says, “And immediately he (Saul) proclaimed Jesus in the synagogues, saying, “He is the Son of God,” (Acts 9:20).
The change wasn’t temporary; it was lifelong. People will sometimes have a “change of heart” when they become afraid or threatened with eternal punishment. Saul, however, repented and remained a faithful child of God even though he was beaten, left for dead and imprisoned (2 Corinthians 11:24).
I thank God he included Saul in the scriptures as an example of conversion. Saul’s obedience strengthens my own faith and resolve and reassures me because of what he had seen and done.