The Solemnity of Sts. Peter and Paul celebrates the two Apostles on which the Catholic Church relied upon in its earliest stage. St. Peter was the first pope and became the unquestioned leader of the Apostles on the day of the resurrection and later Pentecost even though he had denied Christ. He became the Apostle to the Jews in the early Church and was eventually martyred in Rome by upside down crucifixion. 

St. Paul was originally a persecutor of Christianity and had many imprisoned and killed in the interest of preserving the Jewish faith. After his conversion, Paul went to preach to the Jews and Gentiles in Asia Minor through three missionary journeys. His writings to the churches he established are what comprise most of the New Testament. He would eventually be martyred in Rome by beheading. 

We celebrate these two pillars of the Church today because their efforts were key to the proclamation of the Good News to all the world. Through their witness to the Church, they have brought countless souls to know the love of Christ just as they did. Two men who were great sinners, stand as the model as to what the grace of Christ desires to do in your life.