To me, the very least of all saints, this grace was given, to preach to the Gentiles the unfathomable riches of Christ, and to bring to light what is the administration of the mystery which for ages has been hidden in God who created all things; so that the manifold wisdom of God might now be made known through the church to the rulers and the authorities in the heavenly places. This was in accordance with the eternal purpose which He carried out in Christ Jesus our Lord, in whom we have boldness and confident access through faith in Him. Therefore I ask you not to lose heart at my tribulations on your behalf, for they are your glory.
Paul considered himself to be the least among all the saints, not because of false humility, but because of his history. Paul was called directly by the risen Lord, therefore he was legitimately an apostle of Jesus. However, prior to that call Paul was a Pharisee named Saul. And more than that he was a rising star in the world of the Pharisees. See prior to his conversion, Paul was an ardent persecutor of Christ and the church. And more than that Paul, whose name was Saul back then, was making a name for himself by traveling from city to city, to persecute and arrest his fellow Jewish countrymen who had converted to Christianity. Paul was single handedly doing what he could to stop this Christian movement, which he saw as an attack to Judaism. Paul before meeting the risen Christ on the Damascus road, adamantly, with a passion and zeal not match by his fellow Pharisees, sought the persecution of Christians. As a matter of fact, when he was converted he was traveling with letters from the High Priest to Damascus to arrest and persecute Jewish believers. It was on the road to Damascus when he was knocked to the ground and blinded by Jesus. Moreover, Paul was one of the men who had previously consented to the stoning of Stephen, who was stoned for no other cause than being a follower of Christ.
Paul and other men could not refute the arguments of Stephen, so in envy and anger toward Jesus and God, they became furious and stoned him. Paul after his conversion understood that the stoning of Stephen was unjust and without cause. Therefore, it was because of this and because of Paul's previous life as "Saul the Pharisee" that he saw himself as less than all believers. And so Paul explained that if Jesus could choose him, call him and bring him to salvation, then no one is too far away from God for Jesus to reach their hearts and bring them to salvation as well. In addition, to being a trained Pharisee, the Apostle Paul trained under Gamaliel. Gamaliel was a famous Jewish Rabbi, known for his knowledge of Old Testament scripture as well as for his teaching abilities. Paul thoroughly learned and memorized the Old Testament scriptures. Moreover, he was taught directly by the risen Christ Himself. Therefore, Paul more than most was acutely aware of God's perfect righteousness and the unworthiness of fallen humanity. And not only that, but Paul without realizing it had been perfectly trained to argue and refute false doctrine and false pagan religions. Paul, more than most, understood that there is no hope for mankind to save themselves, not even he with all of his religious training could have any hope of entering heaven on his own merit. Paul understood that he was no better than the worst pagan. And Paul knew this deep within in his own heart. When Paul compared his sinful heart to a Holy God, he was acutely aware of his true state, and ours as well. We are all fallen humanity in need of a Savior.