The word apostle means “a sent one”.

Acts 9:3-22 (vs 15) The Lord’s Testimony concerning Paul:

And Saul (Paul), yet breathing out threatenings and slaughter against the disciples of the Lord, went unto the high priest, and desired of him letters to Damascus to the synagogues, that if he found any of this way, whether they were men or women, he might bring them bound unto Jerusalem.

And as he journeyed, he came near Damascus: and suddenly there shined round about him a light from heaven: And he fell to the earth, and heard a voice saying unto him, Saul, Saul, why persecutest thou me?

And he said, Who art thou, Lord? And the Lord said, I am Jesus whom thou persecutest: it is hard for thee to kick against the pricks. And he trembling and astonished said, Lord, what wilt thou have me to do? And the Lord said unto him, Arise, and go into the city, and it shall be told thee what thou must do. And the men which journeyed with him stood speechless, hearing a voice, but seeing no man. And Saul arose from the earth; and when his eyes were opened, he saw no man: but they led him by the hand, and brought him into Damascus. And he was three days without sight, and neither did eat nor drink.

And there was a certain disciple at Damascus, named Ananias; and to him said the Lord in a vision, Ananias. And he said, Behold, I am here, Lord. And the Lord said unto him, Arise, and go into the street which is called Straight, and enquire in the house of Judas for one called Saul, of Tarsus: for, behold, he prayeth, and hath seen in a vision a man named Ananias coming in, and putting his hand on him, that he might receive his sight.

Then Ananias answered, Lord, I have heard by many of this man, how much evil he hath done to thy saints at Jerusalem: And here he hath authority from the chief priests to bind all that call on thy name.

But the Lord said unto him, Go thy way: for he is a chosen vessel unto me, to bear my name before the Gentiles, and kings, and the children of Israel: For I will shew him how great things he must suffer for my name’s sake.

And Ananias went his way, and entered into the house; and putting his hands on him said, Brother Saul, the Lord, even Jesus, that appeared unto thee in the way as thou camest, hath sent me, that thou mightest receive thy sight, and be filled with the Holy Ghost. And immediately there fell from his eyes as it had been scales: and he received sight forthwith, and arose, and was baptized.

And when he had received meat, he was strengthened. Then was Saul certain days with the disciples which were at Damascus. And straightway he preached Christ in the synagogues, that he is the Son of God. But all that heard him were amazed, and said; Is not this he that destroyed them which called on this name in Jerusalem, and came hither for that intent, that he might bring them bound unto the chief priests?

But Saul increased the more in strength, and confounded the Jews which dwelt at Damascus, proving that this is very Christ.

Acts14:14 Luke’s Testimony:

Which when the apostles, Barnabas and Paul, heard of, they rent their clothes, and ran in among the people, crying out, and saying, Sirs, why do ye these things? We also are men of like passions with you, and preach unto you that ye should turn from these vanities unto the living God, which made heaven, and earth, and the sea, and all things that are therein:

Paul’s Testimony Concerning Himself

In all the books Paul authored in the New Testament except two, he spoke of his apostleship: See Romans 1:1-7 & 13; I Corinthians 1:1-3; 9:1-18, 15:1-11; II Corinthians 1:1&2, 12:12; Galatians 1:1-5, 2:6-9; Ephesians 1:1&2; Philippians 1:17 (alluded to); Colossians 1:1&2; I Timothy 1:1, 2:7; II Timothy 1:1; & Titus 1:1-3.

All of Paul’s epistles (letters) authored by Paul were accepted into the canon of Scripture and include Paul’s own statements concerning his calling and appointment by the Lord, thus recognizing Paul’s writings as being from God Himself – “God breathed” as to source (II Timothy 3:16). God makes no mistakes.

Peter’s Testimony II Peter 3:15&16.

And account that the longsuffering of our Lord is salvation; even as our beloved brother Paul also according to the wisdom given unto him hath written unto you; As also in all his epistles, speaking in them of these things; in which are some things hard to be understood, which they that are unlearned and unstable wrest, as they do also the other scriptures, unto their own destruction.

…let the word of Christ dwell in you richly…