Why is Saul of Tarsus considered to be the father of Christianity?

Before he was converted, Saul of Tarsus was a zealous Pharisee hunting down, persecuting, and killing Christians, but he himself was struck down on the road to Damascus, heard the voice of Jesus, became blind, and was healed by a man of God. He was immediately converted to Christianity and changed his name from 'Saul" to "paul'. He then traveled from place to place telling people about Christianity; Paul also sent letters to different churches telling them about this new religion. Paul taught people that Jesus was the son of God and died for the sins of man. Paul also opened the religion to everyone so that even women could even be involved with the ceremonies of the church.

Also, in the New Testament have you ever seen the 1st book of Jesus? 2nd book of Jesus? Of course not. Although the New Testament was written by several of the original 12 disciples, much of it consists of the letters and epistles of Paul written to different churches explaining the working-out of practical Christianity in their day-to-day lives, and in that sense he could be called a father of Christianity.

Also, Paul was an Apostle [or 'one sent forth'] not just a Disciple although they sound the same: an Apostleis one of the leaders of a faith, while a Disciple ['a learner', 'someone's pupil'] is a follower of said faith. However, while not all disciples are apostles, all apostles are disciples because all Christians are disciples:-

Act 11:26 WEB When he had found him, he brought him to Antioch. It happened, that for a whole year they were gathered together with the assembly, and taught many people. The disciples were first called Christians in Antioch.