Paul the Apostle understood his calling to be from and through God and not men. “Paul, an apostle, (not of men, neither by man, but by Jesus Christ, and God the Father, who raised him from the dead ;)” Galatians 1:1
In our text, Paul says his calling was not of or by man, in the Greek the first term “of” focuses on the idea of coming from someone else, the second “by” is more of a channel through which a thing occurs. Paul then is saying his calling to preach Christ among the Gentiles did not come from men nor is it accomplished through the enablement of men.
Paul ministered spiritual truth by Jesus Christ, and God the Father, and the means that God used to enable Paul was the resurrection from the dead.
The Christian is one alive from the dead (dead in sins and trespasses) alive by union with Jesus Christ in His resurrection and victory over sin.