Letters of Paul
Many of the New Testament letters (also known as epistles) are traditionally attributed to the apostle Paul. 1st Thessalonians, 1st and 2nd Corinthians, Galatians, Romans, Philippians, and Philemon are undisputed genuine letters of Paul. There is less certainty about the authorship of 2nd Thessalonians, Colossians, Hebrews, Ephesians, 1st and 2nd Timothy, and Titus. Paul probably wrote 1st Thessalonians and Galatians around 50 A.D., and they are the oldest books in the New Testament.
Paul wrote his letters to various Christian communities to instruct and encourage them in the faith and to address specific problems and disputes that had arisen in those communities. Many of the beliefs and practices of Christianity originated from Paul's teachings in these letters.