Paul, originally a persecutor of Christians for the Roman Empire, received his conversion on the Road to Damascus. It is this that prompted his change of heart, and for his incessant preaching of the gospel. Perhaps driven by his former life, Paul preached the notion of love to all. "If I have no love, I am like a clanging gong," he wrote in one of his epistles. In Galatians, Paul notes the importance of love as one of the 'Fruits of the Spirit'. This devotion to loving one another impacted Christianity as previously, this idea had not been taught so prominently (except, obviously, by Christ). Paul taught that Christians were to, as Jesus taught and demonstrated by His death on the cross, love through all circumstances, as love "never fails". This idea has led to the teaching, in many churches now, of adherents 'becoming love', or becoming one with the qualities Paul so admired in the Saviour he worshipped.
Secondly, Paul's preaching of salvation through faith, and, more importantly, through God's grace, led to the amazing expansion of Christianity through the salvation of new believers. In this way, Paul made Christianity accessible to all, no matter whether they were "Jew or Gentile, male or female, freed or slave". This revolutionised religious expression as anyone knew it, as previously, salvation was earned through works (for example, sacrifices and commandments). Paul, once again building on Jesus' teachings, taught that obeying commandments was not the way to earn salvation, but was a way to show love for Jesus. While this idea was predominately lost until Martin Luther's reformation, Christianity was transformed by this and expanded significantly as a result.
Paul also reiterated the