Next to Christ, who is more eloquent in presenting God's truth and joy and comfort and hope to men than Paul?
The fact that God chose the baddest of the bad to become His greatest human voice is enough to make me a believer. No man could change that much. It must be the work of God alone.
"But he that glorieth, let him glory in the Lord." -- 2 Corinthians 10:17
Concerning certitude and Paul:
For ye have heard of my conversation in time past in the Jews' religion, how that beyond measure I persecuted the church of God, and wasted it: And profited in the Jews' religion above many my equals in mine own nation, being more exceedingly zealous of the traditions of my fathers.
But when it pleased God, who separated me from my mother's womb, and called [me] by his grace, To reveal his Son in me, that I might preach him among the heathen; immediately I conferred not with flesh and blood: Neither went I up to Jerusalem to them which were apostles before me; but I went into Arabia, and returned again unto Damascus.
Then after three years I went up to Jerusalem to see Peter, and abode with him fifteen days. But other of the apostles saw I none, save James the Lord's brother. Galatians 1:11-19
So then neither is he that planteth any thing, neither he that watereth; but God that giveth the increase. I Cor 3:1-7
As Dr. Eckleburg quoted it: