Paul, originally named Saul (Acts 7:58), was a Benjamite Pharisee (Rom 11:1).  He was trained by Gamaliel (Acts 22:3), a then well-known Jewish leader and scholar.  This was almost certainly the same Gamaliel who argued caution in the Sanhedrin (the Jewish ruling circle) against precipitate action towards the Christians (Acts 5:34).  Paradoxically it was Saul/Paul who persecuted them.

Whatever Saul's sins against the early church, one does get the feeling that he knew that he was doing wrong (e.g. Acts 5:5), and was trying to cover up his guilty conscience by action.  This probably explains his rapid conversion - he only had to be 'pushed over the edge', so to speak - although the manner of his conversion undoubtedly had much to do with it.

From the moment of his conversion Saul, renamed Paul, began to preach the crucified Christ.  He then went away for a number of years to study in private, and arrived back on the Christian scene as an energetic apostle.  He conducted two preaching campaigns around Asia Minor, wrote numerous letters, and was eventually incarcerated in the Roman jail in Caesarea.  Here he stayed for two years, the subject of ponderous Roman administration and prevarication by the Governor.

Appealing to Caesar, Paul was conducted to Rome and spent another two years in house arrest at his own expense, awaiting the pleasure of the Roman Caesar.  He used this time to preach further, and several letters were written during his stay in the Italian capital.

His letters span all types of communication, from the intensely personal one to Philemon, through the exhortatory letters to the ecclesias at Philippi and Colossae, to the tightly argued missives to those at Rome and Corinth.  Ignoring the Gospels and the Acts, Paul wrote at least 60% of the remainder of the New Testament.  If we include Hebrews, as I personally think we should, the proportion is over 70%.