"To believe something with absolute certainty, one must start with doubting."  (A quote of the king of Poland in 1766.)  The Apostle Paul, and Lee Stroble--the atheist who wrote, "The Case for Christ" both are highly qualified doubters of the resurrection of Jesus.  Both changed their minds with explosive results in their lives.  They made a 180 degree turn.  (So did the Apostle Thomas.)

Paul was killing Christians with a vengeance.  He was traveling from city to city stoning them to death.  Those who believed in the resurrection of Christ were his targets.  Paul thought he was doing God's work by killing heretics who threatened the Jewish religion.  Acts 8:3 "As for Saul (Paul), he made havoc of the church, entering into every house, identifying men and women, and committing them to prison."  Rather like the caliphate in the middle East today--which says if you aren't Muslim, then they will kill you.  Death by religion.

Paul, "...yet breathing out threatenings and slaughter against the disciples of the Lord, went to the high priest...", for a letter to give him authority to travel to other cities and arrest people. Acts 9:1.  But on the way to Damascus, Paul met Christ.  And was forever changed.  Now he had to convince the disciples that he was a different, new, born again disciple of Christ.  The Apostle to the Gentiles.

Peter, James and John didn't trust Paul when said he was a Christian--much less an Apostle.  And they were certainly justified.  Acts 9:26 "And when Saul was come to Jerusalem...to join himself to the disciples...they were all afraid of him and didn't believe that he was a disciple."  But Barnabas, a loving Christian man, vouched for Paul.  Then, Paul explained that God had called him to witness to the Gentiles--which was strange to the disciples.  They believed Jesus was a Jewish Messiah and they had no concept of God including Gentiles into the mix.  Who did this man, Paul, think he was!!

Paul had everyone against him at this point.  But he did not let it deter him.  He began to preach God's message to the Gentile peoples, and write prolifically to encourage them to stand fast and not be discouraged.  And we have those letters.  An eye witness account as to what happened in Paul's life.  So when he writes to the Ephesians and says,"..he has chosen us in him before the foundation of the world that we should be holy...having predestinated us unto the adoption of children by Jesus Christ," Eph. 1: 4-5, Paul was telling everyone that God wanted us.  He chose us.  Jews and Gentiles.