To be called an apostle you must be personally chosen by Jesus. When Jesus walked amongst us in the world, he chose twelve, each symbolizing the tribes of Israel. One day though there was a man that was prosecuting members of the early church, his name was Saul. He had an encounter though that would change his life. One day Jesus appeared to him in person and called him to be his apostle to the gentiles. He took a new name that day, and became known to all as Paul the thirteenth disciple of Jesus.
This to me, made those gentiles that embraced Christianity, the thirteenth tribe of Israel. Thus making the number thirteen a lucky number.