Paul had not met Jesus in person. Jesus never preaches changes to Law. He comes to fulfill it. Jesus has completed the Father's will on earth. Why Paul then?
- Commissioning. Paul saw the risen Christ on the road to Damascus (as recorded in Acts 9, 22, and 26). Christ commissioned him personally. Then he repeated that commission to a faithful believer as a second witness:
13 “Lord,” Ananias answered, “I have heard many reports about this man and all the harm he has done to your holy people in Jerusalem. 14 And he has come here with authority from the chief priests to arrest all who call on your name.”
15 But the Lord said to Ananias, “Go! This man is my chosen instrument to proclaim my name to the Gentiles and their kings and to the people of Israel. 16 I will show him how much he must suffer for my name.” (Acts 913-16)
- Teacher. Paul says he received his message directly from Jesus, not from men.
11 I want you to know, brothers and sisters, that the gospel I preached is not of human origin. 12 I did not receive it from any man, nor was I taught it; rather, I received it by revelation from Jesus Christ. (Galatians 1:11-12)
- Endorsement. Peter affirms Paul's message and equates it with Scripture:
15 Bear in mind that our Lord’s patience means salvation, just as our dear brother Paul also wrote you with the wisdom that God gave him. 16 He writes the same way in all his letters, speaking in them of these matters. His letters contain some things that are hard to understand, which ignorant and unstable people distort, as they do the other Scriptures, to their own destruction. (2 Peter 3:15-16)
- Declaration. Paul calls himself an apostle in his writings. If his words are Scripture, they cannot lie. One place he calls himself an apostle is in 1 Thessalonians 2:6. Another is in Romans:
1 Paul, a servant of Christ Jesus, called to be an apostle and set apart for the gospel of God— 2 the gospel he promised beforehand through his prophets in the Holy Scriptures 3 regarding his Son, who as to his earthly life was a descendant of David, 4 and who through the Spirit of holiness was appointed the Son of God in power by his resurrection from the dead: Jesus Christ our Lord. (Romans 1:1-40)
- Suffering. Being with Jesus, being taught by him, teaching others effectively about Jesus, originating new Scripture, and performing signs and miracles were all marks of an apostle that Paul possessed - plus one more. The greatest prophets and saints endured trials and persecutions, like Job, David, Elijah, Hosea, Jeremiah, and all the Apostles. How much did Paul suffer for the name?
3 We put no stumbling block in anyone’s path, so that our ministry will not be discredited. 4 Rather, as servants of God we commend ourselves in every way: in great endurance; in troubles, hardships and distresses; 5 in beatings, imprisonments and riots; in hard work, sleepless nights and hunger; 6 in purity, understanding, patience and kindness; in the Holy Spirit and in sincere love; 7 in truthful speech and in the power of God; with weapons of righteousness in the right hand and in the left; 8 through glory and dishonor, bad report and good report; genuine, yet regarded as impostors; 9 known, yet regarded as unknown; dying, and yet we live on; beaten, and yet not killed; 10 sorrowful, yet always rejoicing; poor, yet making many rich; having nothing, and yet possessing everything. (2 Corinthians 6:3-10)
And a second time:
23 Are they servants of Christ? (I am out of my mind to talk like this.) I am more. I have worked much harder, been in prison more frequently, been flogged more severely, and been exposed to death again and again. 24 Five times I received from the Jews the forty lashes minus one. 25 Three times I was beaten with rods, once I was pelted with stones, three times I was shipwrecked, I spent a night and a day in the open sea, 26 I have been constantly on the move. I have been in danger from rivers, in danger from bandits, in danger from my fellow Jews, in danger from Gentiles; in danger in the city, in danger in the country, in danger at sea; and in danger from false believers. 27 I have labored and toiled and have often gone without sleep; I have known hunger and thirst and have often gone without food; I have been cold and naked. 28 Besides everything else, I face daily the pressure of my concern for all the churches. 29 Who is weak, and I do not feel weak? Who is led into sin, and I do not inwardly burn?
30 If I must boast, I will boast of the things that show my weakness. (2 Corinthians 23-30)
- Power. Paul's words continue to demonstrate the power of God. 1 Corinthians 13 taught me what true love is, and enabled me to make a personally costly decision to help another person, turning me away from a selfish life. Memorizing Galatians 2 took away my fear of death in a matter of weeks. Philippians 2 showed me the path out of my long term depression into a life of joy. Thus three of the biggest spiritual breakthroughs in my life came from meditating on Paul's words. If he is not an Apostle, who is?
NOTE: Another recent question deals with aspects of this issue: How did Paul learn the gospel?