"Paul, an apostle not from men nor through the agency of man, but through the agency of Jesus Christ and God the Father who raised him from the dead . . ." - Gal 1.1.
Here Paul affirms that the origin and agency of his apostleship (i.e., the status of being 'sent-by-God-to-preach') rests in Jesus Christ and God the Father rather than mankind. He will later emphasize that the apostolic message itself also has come through the agency of Christ (1:12). As a result, Paul does not seek to please mankind, but rather to be a faithful slave of Christ (1.10). Further, the Gospel message itself is not 'according to man' (1.11), but it has been received through Christ (1.12).
Paul's argument serves to compel these 'foolish Galatians' to be faithful to the true Gospel - the apostolic message received from Christ (and his apostles). But in Galatia, some Jews have perverted this message. Their message is according to man; their 'sent-ness' is from and through man.
In just one verse, Paul touches on the nature of his apostleship (a part of which is the content of the apostolic message) - that it is from God/Christ, not man. He also mentions a core element of the message: the resurrection of Christ by God the Father. As he moves through the next few verses, Paul summarized the apostolic ('from/through Christ') message as follows: Resurrection (1.1); Personal atonement (1:4, note 'us' = personal element); Subsequent rescue (1.4); Accordance with God's will (1.4); Result in God's ongoing glory (1.5).
Fascinating. So much is packed into these first five verses!