The list of apostles in the Bible is short. The twelve were carefully chosen from among a crowd of disciples who were following the Lord Jesus in his earthly ministry. They were called to leave their jobs, sell all they had, and follow the Lord into the kingdom (Luke 18:22).
They were obedient to the call to show fruits meet for repentance and to be water baptized (Mat 3:11). The apostles were to lead the remnant nation ‘bearing forth the fruits thereof’ into the kingdom (Mat 19:28; Mat 21:43).
It was because they accepted the Messiah that they were given the great opportunity to be witnesses of his resurrection.
But Paul, who was then named Saul, did not bring forth worthy fruits of repentance. Nor was he a disciple of Jesus in his earthly ministry. When Christ talked about blind guides and hypocrites he could have very well have been speaking to Saul of Tarsus.
Paul confesses that he was a blasphemer who denied the Lord, a persecutor of the true apostles, and injurious towards their disciples (1 Tim 1:13).
Something unprecedented happened on the road to Damascus. An apostle was commissioned purely by the grace of God to go to the Gentiles. He was not chosen by man, nor taught by man, nor was he evaluated by the same standards as the twelve before him.
The manifestation of God’s grace had begun to flow by the merits of the cross of Calvary. The twelve held credentials of being disciples of the Lord since the beginning of Jesus’ earthly ministry and had given up everything for the Lord. But just one man was chosen solely by God’s grace despite his unfruitful unbelief to be an apostle of Jesus Christ to the Gentiles (Rom 11:13).