1:1 Paul, an apostle (not from men nor through man, but through Jesus Christ and God the Father who raised Him from the dead), 2 and all the brethren who are with me, To the churches of Galatia.
When Paul was writing to churches that did not question his authority, he would begin his letters by saying, “Paul, a bondservant of the Lord.” On the other hand, when there were questions about his apostolic authority, or problems in the church internally, he would say “this is Paul the Apostle.” This would set the tone for the letter, and also validate his calling and his authority right up front.
Paul's enemies would try and discredit his ministry. They would question his apostolic authority. They would suggest that Paul wasn't a genuine apostle. They would seek to poison the minds of those Paul had led to Christ, by raising false accusations. This became increasingly difficult to defend when Paul was put in prison.
Definition: Apostle: means an appointed representative with an official status who is provided with the credentials of his office.
Although we are not called apostles today, we are still called to represent Jesus to this world. The credentials we have come directly from the Creator who said, "GO into all the world and preach the gospel."