The influential figure behind the spread of Christianity in the Roman Empire was the Apostle Paul. Initially a persecutor of Christians, Paul underwent a profound conversion experience and became one of the most pivotal figures in early Christianity. His missionary journeys across the Mediterranean established Christian communities and spread its teachings beyond Jewish populations to Gentiles. Paul wrote numerous letters, or Epistles, addressing theological issues and providing guidance to early Christians. His writings contributed significantly to the New Testament and the development of Christian doctrine, establishing the foundations for Christianity's growth and its eventual adoption as the state religion of the Roman Empire.