Paul of Tarsus, originally Saul or Saulus. Saul was a pharisee studying in Jerusalem who began as a persecutor of the first Christians and was converted by a vision of Jesus Christ as he was on the road to Damascus to prosecute Christians in that city.
According to the book of Galatians in the Bible, Paul first went to Arabia and then spent three years in Damascus. After this period he travelled to Jerusalem where he met James, one of the apostles.
He then began the first of his three apostolic travels through Syria, Cicily, Turkey, Cyprus, Crete and Greece. He preached Jesus Christ to be the crucified and risen Son of God to Jews in synagogues and to the 'gentiles' in villages and cities. He started churches wherever converts could gather to study the Jewish scriptures and in Ephesus on the Turkish west coast he even stayed two years to teach and strengthen the new christians (according to the book of Acts chapter 19).
Paul usually chose one or more companions for his travels. Barnabas, Silas, Titus, Timothy and John Mark were part of his team. Titus he left at Crete to help strengthen the new church there. Paul was not only a theological scholar but also a tentmaker, by which he earned his money. He organised the raise of money for victims of a famine in Palestine (details?).
Paul was persecuted many times. He suffered detention in Philippi, was lashed and stoned several times and almost murdered once. He caused a great uproar in the theatre in Ephesus, where local silversmiths feared loss of income due to Paul's activities. Their income relied on the sale of silver statues of the goddess Arthemis, whom they worshipped.
Paul was born a Roman citizen; he used that status to appeal his conviction to Rome and spent the last year of his life in transit to Rome and in detention there.
Paul wrote many letters to christian churches and to some individuals. Some of those letters (with internal evidence of Paul's authorship) have been preserved and are part of the New Testament canon. The authorship of the letter to the Hebrews is a question of debate. The letters he wrote from captivity are called the 'prison-letters', and were probably written in Rome.