Galatians 1:1-2 (ESV)
Paul, an apostle—not from men nor through man, but through Jesus Christ and God the Father, who raised him from the dead—  and all the brothers who are with me,
To the churches of Galatia:
Paul took his apostleship seriously. He had to defend it. His message that we are saved by grace through faith, is astonishing. It we were to question his apostleship, then we would not have the assurance of salvation. It is quite simply that simply. Paul spends much time defending his apostleship in this letter. He was sent by Christ, not by men. Christ Himself called Paul to be an apostle that is to speak for Him, and on His behalf. Paul here is telling the Galatians that to question him is to question Christ. This gospel he proclaims, the forgiveness of sins, it is Christ’s Gospel. You can trust it.