Apostle of Jesus Christ
Paul identified himself af a servant of God and an apostle of Jesus Christ. As servant, Paul belonged to God---he was "tied" to God and committed to service and obedience. As an apostle, Paul was "set apart", sent out or commissioned by Jesus to preach.
Paul, a servant of God, and an apostle of Jesus Christ, according to the faith of God's elect, and the acknowledging of the truth which is after godliness...(Titus 1:1)
The word, "apostle" comes directly from the Greek word, "apostolos", meaning a delegate or ambassador. It comes from two combined words meaning set apart, or sent out. The two words literally mean "set fast away", where "fast" means secure, or unshakeable. An apostle was unshakenly commissioned to be God's ambassador.
Paul was called apostle because Jesus had commanded him to preach the gospel, the good news of salvation.
But hath in due times manifested his word through preaching, which is committed unto me according to the commandment of God our Saviour; (Titus 1:3)
Paul was a sort of apostle even before being commissioned by Jesus. He was a self-proclaimed "enforcer" for the radical Jews that hated Jesus.
And Saul, 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. (Acts 9:1-2)
But Paul's heart was dramatically converted, and he became an apostle of Jesus Christ.
...for he is a chosen vessel unto me, to bear my name before the Gentiles, and kings, and the children of Israel:(Acts 9:15)
As they ministered to the Lord, and fasted, the Holy Ghost said, Separate me Barnabas and Saul for the work whereunto I have called them. And when they had fasted and prayed, and laid their hands on them, they sent them away. So they, being sent forth by the Holy Ghost, departed unto Seleucia; and from thence they sailed to Cyprus. And when they were at Salamis, they preached the word of God in the synagogues of the Jews:(Acts 13:2-5)
Paul was a "late-comer" as an apostle. Jesus had appointed twelve men early in His ministry, including Judas, who would later betray Him. These original apostles (except Judas) followed Jesus, saw Him crucified, and witnessed His resurrection. Paul did not become a Christian until well after the Church had begun to grow.
There is some indication that Paul encountered opposition to his being considered one of the apostles.
If I be not an apostle unto others, yet doubtless I am to you: for the seal of mine apostleship are ye in the Lord. (1 Corinthians 9:2)
Paul himself was humbled by his commission as apostle, but he did not doubt it.
And last of all he was seen of me also, as of one born out of due time. For I am the least of the apostles, that am not meet to be called an apostle, because I persecuted the church of God. But by the grace of God I am what I am: and his grace which was bestowed upon me was not in vain; but I laboured more abundantly than they all: yet not I, but the grace of God which was with me.(1 Corinthians 15:8-10)
But no matter what groups disputed with Paul, or how humble he felt about being counted worthy by Jesus to preach for Him, the other apostles fully accepted Paul.
And when James, Cephas, and John, who seemed to be pillars, perceived the grace that was given unto me, they gave to me and Barnabas the right hands of fellowship...(Galatians 2:9)
I think we all feel unworthy at times...we think back to things we're ashamed of, and we might wonder, Can God really love me? We need to remember all His promises, and realize that, like Paul, "God so loved us, that He gave His only begotten Son, that whosoever believes in Him should not perish but have everlasting life."
God is glorified by the mercy He shows to those He forgives.