A true apostle had the power to performed signs, wonders, and miracles. Paul said, "I have become foolish; you yourselves compelled me. Actually I should have been commended by you, for in no respect was I inferior to the most eminent apostles, even though I am a nobody. "The signs of a true apostle were performed among you with all perseverance, by signs and wonders and miracles." 2 Corinthians 12:11-12. NASB
A true apostle was a witness of Christ's resurrection. Paul saw Christ and was a witness of his resurrection 1 Corinthians 9:1&2; 15:4-10. A true apostle was chosen by Jesus Christ. Paul was chosen by Jesus in Acts 9:15-17.
By these passages Paul's apostolic authority is confirmed. The words that Paul spoke concerning the gospel are forevermore confirmed in those who believe. ". . . how shall we escape if we neglect so great a salvation? After it was at the first spoken through the Lord, it was confirmed to us by those who heard, God also bearing witness with them, both by signs and wonders and by various miracles and by gifts of the Holy Spirit according to His own will." Hebrews 2:3-4. NASB