But Saul was going everywhere to destroy the church. He went from house to house, dragging out both men and women to throw them into prison. - Acts 8:3
Even before Saul was a part of the kingdom of God, he was working for it unwittingly. By attacking Christians, he caused them to spread out geographically. If he had not done this, it is entirely possible that the believers would have remained in a holy huddle in Jerusalem. But Jesus had told them to "go and make disciples of all the nations" (Matthew 28:19). Then along came Saul to arrest Christians, so they dispersed to various areas, and the gospel spread.
Saul was living off hatred and threats, like a wild beast stalking its prey. He was on a search not for God, but for the followers of God. A raging fury obsessed him. And if someone would have said to him as was leaving Jerusalem, "Guess what, Saul? Before the day is over, you will be a follower of Jesus," he would have laughed out loud. But that is exactly what happened. And his conversion was so unexpected that many greeted it with skepticism and suspicion.
Saul's conversion was such an unlikely event that Baron George Lyttelton, a British agnostic of the 18th century, thought it would not be too difficult to disprove. In doing so, he reasoned, he could then show that the rest of the New Testament was unworthy of credibility. But after careful research on the life of Saul of Tarsus (later to become Paul), Lyttelton was convinced the story was true and became a Christian himself. He concluded, "I thought the Conversion and the Apostleship of St. Paul alone, duly considered, was of itself a demonstration sufficient to prove Christianity a Divine Revelation."
So when some people turn a deaf ear to the gospel, put us down, or attempt to shut us up, remember that the Word of God will not return void.