"Paul, an apostle of Jesus Christ by the will of God..."
Paul was an Apostle and opened his letters with his name and his office in the church. Particularly he was the Apostle to the gentiles and thank God that we have been grafted into the promises of God and are lacking in no area. We have the Holy Script, the Messiah, and even an Apostle who carried the New Testament revelation of Jesus Christ to the gentile world. He stated this position as an accreditation of his authoritative teachings. No serious Christian refutes his apostleship but, if you were curious, a study into the life of Paul would show you that all the necessary signs of an Apostle were wrought in him. Why is the office of the Apostle so important? Because according to Ephesians 2:20 the church, the habitation of God through his Spirit, is built on the foundation of the Apostles and Prophets, with Jesus Christ as the chief cornerstone. We know that Jesus is front and center and the very object of the Christian faith, and while the Apostles don’t outshine Him, they did shine out for him. Jesus had hand-selected him on the road to Damascus and inspired him to write Holy scriptures and all his messages and ministry were confirmed by signs and wonders necessary in that time to confirm the message and messenger.
As an Apostle of Jesus Christ by the will of God, he is a valid candidate for God’s authoritative revelation. I mean he is not only validated to the Colossian church but to every church in the whole World. His being an Apostle means that the revelation and instruction given to him by the Holy Ghost is authoritative to the church today. This office was foundational and when the foundation was completed, The Bible, the office, and the confirming signs of the office ceased. God’s word was always accompanied by signs and wonders during the periods that it was being given to his special officers, like Apostles, Prophets, and Moses. As new revelation from God was given to these officers, they would be coupled with miraculous power to confirm the message and messenger. Jesus’ message and ministry were confirmed by signs and wonders [Joh 3:1, Mat 11:2-5, Acts 2:22] The Apostles' message and ministry was confirmed by signs and wonders [Mark 16:20, Acts 4:33, Acts 14:3, Hebrews 2:3-4] but when the revelation was complete, the recipients of the revelation were expected to live their life by the message that had been given and confirmed by miracles. Take Israel for example, In the days that God was giving revelation to Moses, and he was penning down the first five books of the Bible, they experienced miracle after miracle. The cloud by day, the fire by night, and the water from the rock were miracles that were necessary for their living. Then as soon as Moses dies, and the five books are completed, the book of Joshua opens and their success is not rooted in miracles but in the Word of God that had been previously shown to them in the books of Moses. Their success was promised in their adherence to the revealed and confirmed word of God.
Joshua 1:7-8 7 Only be thou strong and very courageous, that thou mayest observe to do according to all the law, which Moses my servant commanded thee: turn not from it to the right hand or to the left, that thou mayest prosper whithersoever thou goest. 8 This book of the law shall not depart out of thy mouth; but thou shalt meditate therein day and night, that thou mayest observe to do according to all that is written therein: for then thou shalt make thy way prosperous, and then thou shalt have good success.
This same principle can be seen in the prophets and the Apostles' ministry, the word of God was given and confirmed and now our success is rooted in adherence to the revealed, confirmed word of God given to us by these officers, not in a looking for miracles and signs whose purpose has been completed in the confirmation of God's Word. Study the life of Paul and you will see his ministry, calling, and message are all confirmed, he is “An Apostle of Jesus Christ by the will of God” and we should become familiar with and obedient to the Word of God given through this Apostle.