The most important change that Paul made was to carry Christianity to the Gentiles, that is the non-Jewish population. All early Christians, including Paul, had been Jewish; and the common belief was the Christianity was a new, reformed form of Judaism. Paul carried the new religion to non-Jews and found many converts among them.
Secondly, Paul transformed Christianity into a missionary religion. It previously was a closed religion which allowed some new members, but did not reach out to others. Paul was among the first to carry the religion to distant areas, in keeping with Jesus' command to his disciples:
He said to them, “Go into all the world and preach the good news to all creation. Whoever believes and is baptized will be saved, but whoever does not believe will be condemned.
Thirdly, Paul removed Christians from obedience to the Jewish law. It had previously been taught that all Christians must observe Jewish dietary law and custom, including the circumcision of males. Paul taught that the law had been for the purpose of atoning for sin; however the sacrificial death of Jesus had paid the price of sin, therefore Christians were no longer bound by the law. He stated in Romans 8:1
Therefore, there is now no condemnation for those who are in Christ Jesus, because through Christ Jesus the law of the Spirit who gives life has set you] free from the law of sin and death. For what the law was powerless to do because it was weakened by the flesh,] God did by sending his own Son in the likeness of sinful flesh to be a sin offering. And so he condemned sin in the flesh, in order that the righteous requirement of the law might be fully met in us, who do not live according to the flesh but according to the Spirit.
paul the apostle was the most successful of all the apostles of Jesus christ. First, paul made it possible for the gospel to spread to other parts of the world as commanded by Jesus, through his missionary journeys.
Second, paul broke the partition wall between the Jews and Gentiles by forbearing the message that Jesus died for all people whether jew, greek, roman or others.
third, paul was a learned man and so his literacy potential was manifested in his various "EPISTLES" or letters to churches of various communities and backgrounds in his days.
finally, apostle Paul was instrumental to disciplining of many younger christian converts like ' TIMOTHY, TITUS, PHILEMON, JUDE, etc, who took over the work from him commitedly and steadfastly, making it possible for the world to have the gospel of Jesus Christ today.