Saul the Christ-hating Pharisee (see Was Paul Among Them?) was chosen, by Jesus Christ, to serve Christ, as an apostle, later known as Paul, after his eyes were opened (see Paul's Blindness Lesson). Christ may have chosen Paul because he was a Pharisee, a man well-familiar with the Holy Scriptures - and therefore also able to read and write (which most people could not do back then) which Paul would put to good use as a writer of most of the Epistles.
But Paul had something else that was vital to what he would be sent to do - tenacity, an undefeatable determination of purpose (not to be confused with carnal mule-headed stubbornness), to do whatever he set out to do, whether persecuting Christians, or, later, serving Christ as a Christian himself. Paul's life of overcoming adversity is an example for all Christians.
"I have fought a good fight, I have finished my course, I have kept the faith"
The LORD appointed Paul to serve as an apostle, a word that merely means someone who is sent. Paul was sent into a hostile world, in the service of Christ and His people (see Paul's Ministry), that would require an iron will to endure, "9:16 For I will show him how great things he must suffer for my name's sake" (Acts 9:16 KJV).
"9:15 But the Lord said unto him, Go thy way: for he is a chosen vessel [see also Treasure In Earthen Vessels] unto me, to bear my name before the Gentiles, and kings, and the children of Israel: 9:16 For I will show him how great things he must suffer for my name's sake."
For the rest of his life, Paul endured rejection by his own people, beatings, imprisonments, shipwreck, hunger, thirst, cold, and in the end, execution - all for the "wrong" (according to an evil, hypocritical world that, for the most part, doesn't yet know true right from wrong; see A Brainwashed World) of preaching the LORD's Truth.
"11:24 Of the Jews five times received I forty stripes save one. 11:25 Thrice was I beaten with rods, once was I stoned, thrice I suffered shipwreck, a night and a day I have been in the deep; 11:26 In journeyings often, in perils of waters, in perils of robbers, in perils by mine own countrymen, in perils by the heathen, in perils in the city, in perils in the wilderness, in perils in the sea, in perils among false brethren; 11:27 In weariness and painfulness, in watchings often, in hunger and thirst, in fastings often, in cold and nakedness. 11:28 Beside those things that are without, that which cometh upon me daily, the care of all the churches." (2 Corinthians 11:24-28 KJV)
As he saw his martyrdom nearing (see Martyrs), Paul's farewell was not a good bye, but a command for all of those who came later to carry on where he and the other apostles who endured the same (e.g. see Peter's Prophecy) left off: "4:2 Preach the word; be instant in season, out of season; reprove, rebuke, exhort with all longsuffering and doctrine" (2 Timothy 4:1-8 KJV)
"4:1 I charge thee therefore before God, and the Lord Jesus Christ, who shall judge The Quick and The Dead at his appearing [see also Could Christ Return Tonight?] and his kingdom; 4:2 Preach the word; be instant in season, out of season; reprove, rebuke, exhort with all longsuffering and doctrine. 4:3 For the time will come when they will not endure sound doctrine; but after their own lusts shall they heap to themselves teachers, having itching ears; 4:4 And they shall turn away their ears from the truth, and shall be turned unto fables. 4:5 But watch thou in all things, endure afflictions, do the work of an evangelist, make full proof of thy ministry.
Fact Finder: What good does adversity have for Christ's people?