Why does Paul call himself the least of the apostles?

Les explains that Saul of Tarsus had received authority from the chief priests, and when believers were put to death, he gave his voice or vote against them. When they were brought before the religious leaders and the vote was whether to put them to death or in prison, Saul voted to put them to death, thinking, “Why feed an enemy? Get rid of him. Take no prisoners.” He punished them often in every synagogue and compelled them to blaspheme, forcing people to blaspheme Jesus Christ, renege on him, recant faith in him, or be put to death. Les says this was the ploy of persecutors down through the ages: force people to make a choice. Saul put his fellow Jews into that same position. After he found out that the one he was persecuting was really his Old Testament Jehovah, it beat him down, and he never could quite get over it. He sorrowed for the day he had so persecuted these Jewish believers. So in 1 Corinthians 15 he says he is the least of the apostles because he persecuted the church, or the assembly of God, those Jewish believers at Jerusalem. Then verse 10 begins with “But.”