The Ninevites were a murderous, brutal people, who terrorized the Middle East for centuries. They were rightly named the "bloody city" (Na 3:1). The Jewish people would probably have considered the Ninevites the people least likely to convert. Yet all 120,000 people in the city of Nineveh converted at the prophecy of Jonah (Jon 3:6; 4:11).
Saul was an accomplice in the murder of the first martyr, Stephen (Acts 7:58). Then Saul got worse. He "began to harass the Church. He entered house after house, dragged men and women out, and threw them into jail" (Acts 8:3). Saul admitted he was "a blasphemer, a persecutor, a man filled with arrogance" (1 Tm 1:13). To the persecuted Christians of the early Church, Saul was the least likely to convert. But he did convert and even became an apostle, one of the first Christian missionaries, and author of several books of the New Testament.
The first Christians were long persecuted and martyred by Rome and its emperors, especially by the brutal Nero and Domitian. Who would have thought that the Roman emperor, Constantine, would convert to Christianity and lead Rome to Christ?
Dr. Bernard Nathanson was the foremost abortionist in the USA. He even propagandized the country and the U.S. Supreme Court to get abortion legalized. Who would expect Nathanson and several other anti-life mass murderers and serial killers to give their lives to Jesus?
In His almighty power and merciful love, Jesus converts the "un-convertible." Live, act, and pray accordingly.