Read Acts Chapter 9 verses 1-18 for the details.

By birth, heritage, and training, Saul of Tarsus was a proud, and intense Jewish Pharisee whose convictions led him to become the main persecutor of the early Christian church, see e.g. Acts 22:4.

But God spoke to Saul in a dramatic way at a time when he was breathing out “threatening and slaughter” (Acts 9:1) which resulted in him completely changing his mind about the most important things in his life. The main issue in his mind was, “what is God’s way of making us right with Himself?” Traditionally, Saul had been taught, and he fervently believed, that rightness with God was achieved through keeping God’s law. This was something that was dependent on his own goodness and his ability to obey God’s law. That was why Saul was so fervent a member of the Pharisees, who demand the strictest obedience to the Jewish law (Phil 3:5). But when he grasped that Jesus was the Son of God who had died on the cross as a sacrifice for sin, he recognized that instead of working to secure his own salvation (an impossibility!), he should trust Christ to save him. He himself testified, “what things were gain to me those I counted loss for Christ (Phil 3:7). Salvation, he recognised, was not based on his own efforts but on Christ’s action. Not surprisingly, Saul renounced a religion that ignored Christ’s sacrificial death and resurrection, and he embraced with all his heart the living Lord Jesus and His cause.

When a man is truly converted to Christ, his life is changed and with God's help he changes his world. The Lord Jesus said, 'Except ye be converted...ye shall not enter into the kingdom of heaven' (Mat 18.3).