Why was Saint Paul a saint?
As one of the most influential Church Fathers and one of its earliest martyrs, it is not surprising that Paul (born Saul) was canonized by the Catholic Church (Paul was never formally canonized, since formal canonization began in 993 with Pope John XV). Despite being of of Christianity's most fervent persecutors, Paul converted to Christianity after Christ appeared to him on the road to Damascus (and changed his name from Saul to Paul).
Now a Christian, Paul embarked on several missionary journeys to spread the Gospel. He performed miracles throughout these journeys, including casting out demons. He and his companions continually faced imprisonment and torture from those hostile to Christianity. Paul also wrote fourteen letters which have been deemed canonical by the Church (that is, written under the inspiration of the Holy Spirit and worthy to be included in the Bible). Finally, Paul was martyred for his faith; he was beheaded in 67 A.D. For these reasons, the Catholic Church considers Paul a saint and celebrates his feast day on June 29.
According to Catholicism, Paul is considered one of the most influential apostles.
Paul -- known throughout his early life as Saul -- was originally one of the most dangerous men to the early Christian church. He was relentless in his efforts at wiping out the followers of Christ, and was present at the stoning of Stephen, another early apostle. (Acts 7:58 & 8:1) He personally hunted, imprisoned, and killed early Christians.
On the road to Damascus, God Himself spoke to Paul -- asking, "Saul, Saul, why do you persecute Me?" (Acts 9:4) -- blinding him for three days. In a vision, another man named Ananias was told by God to go to Paul and lead him in his conversion. Obviously Ananias was skeptical, but he obeyed, and Paul became one of the most fervent believers from that time on.
He is best known for his extensive missionary journeys, primarily to the Gentiles (he attempted preaching to the Jews first, but when they rejected his words, he moved on as instructed). He spent his life telling the good news of Jesus' teaching, death, and resurrection, and spending much of his life beaten and imprisoned because of it. (How ironic, right?)
Although he is one of the most "famous" of the apostles, he himself was hesitant to accept praise, calling himself the "chief among sinners." (1 Timothy 1:15) He knew what Christ had done for him personally, and how he had changed as a result of God's love, and his only mission was to share this new truth as far as he could take it.
Because of his missionary journeys and writings, he is Catholicism's patron saint of missionaries, writers, and tentmakers (his prior profession), among other things, according to catholic.org. His feast day is shared with St. Peter, but he is celebrated on several other days throughout the year for various key moments in his life.