There is something important that Paul emphasised much towards the end of his life: "Look at my example", he says, "see how I lived" This is what he tells Timothy repeatedly. "I thank Christ Jesus our Lord who has strengthened me because He considered me faithful, putting me into the ministry" (1 Timothy1:12).
Even though God sovereignly calls people to His ministry, He tests a man before He puts him into the ministry. Paul was tested for a period of 10 years (from Acts 9 where he was converted to Acts 13 where he was sent out into the ministry). What was happening to Paul those 10 years? God was watching Paul to see whether he was faithful in his daily life as a Christian. After God tested him for 10 years and saw his faithfulness, then He said, "Now I will send you out to serve Me."
If God calls you He will first test you. I know that God tested me for 16 years (9 of them in full-time Christian work), before He opened up the ministry He had for me - to build new-covenant churches. Don't imagine that as soon as you step out into Christian work, you will have an anointed ministry. God will test you to see whether you are faithful in the little things. If you are faithful in little things, faithful to study His word, faithful with money, faithful with time, faithful in purity, upright, honest, and humbling yourself under the authorities God places over you, then, after 10 years perhaps, He will put you into an effective ministry. When God Himself puts you into a ministry, you will do more in one year than when doing ministry according to your own ideas in 30 years. Paul says, "He (Christ) put me into the ministry" (1 Timothy1:12). It wasn't any man who ordained Paul. " Even though I was a blasphemer, a persecutor" (1 Timothy1:13).
God puts converted sinners who were once wicked into the ministry. That is an encouragement to all who have lived a very wicked life in the past. In one of the writings that have come down from the 2nd century, the author says, that Jesus Christ, in order to show that He came not to call the righteous but sinners to repentance, picked up some of the worst sinners of His time to become His apostles. Paul was one of them. "Christ Jesus came into the world to save sinners among whom I am the greatest" (1 Timothy1:15).
So God doesn't necessarily pick up those who have been brought up in God-fearing homes and who have lived clean lives always. He does use many like that. But He also picks up those who have lived in the depths of sin. He picks up adulterers, thieves, drunkards and drug addicts, and makes them apostles, to prove that sin in one's unconverted life doesn't disqualify anyone from even becoming an apostle. Paul, the greatest sinner became the greatest apostle. God can do a lot through any of you, if you will repent of your sins and trust Him.