Paul, an apostle, (not of men, neither by man, but by Jesus Christ, and God the Father, who raised him from the dead;)
Note 1 at Ga 1:1: Paul is a Latin name that means "small, little" (Smith's Bible Dictionary). It is used first in Ac 13:9. It was probably the name used to identify him more closely with the Gentiles, since this was a non-Jewish name. Paul could have been small in stature, or he may have wanted to identify himself as small in comparison to the magnified name of Jesus that he was preaching.
Note 2 at Ga 1:1: The word "apostle" is a transliteration of the compound Greek word "APOSTOLOS." APOSTOLOS is comprised of "APO" ("from") and "STELLO" ("to send") (Vine's Expository Dictionary), thus referring to the act of sending someone on a commission to represent the sender. It was used of a messenger who was provided with credentials. Our word "ambassador" would be a good translation ("Galatians in the Greek New Testament" by Kenneth S. Wuest, p. 28).
It was important that Paul defend his apostleship, for if the false teachers, called Judaizers, could have discredited his apostleship, they would have also discredited the message he was preaching. Therefore, in the very first verse, Paul established his apostleship as the authority from which he spoke.
Note 3 at Ga 1:1: The point Paul was making is that his calling was divine. He didn't call himself, nor did any other person or group. God Himself called Paul to his ministry. Therefore, those who resisted Paul were resisting God, who sent him.
Paul was saying these things to prepare the Galatians so that they would not take what he was about to say lightly. He was going to be very blunt with them, even to the point that they could have been offended very easily (Ga 4:16). But Paul wanted them to know that he was speaking God's heart, not just his own heart. Hopefully, this would cause them to think twice before dismissing Paul's rebukes. As Paul said in 1Th 2:13, "When ye received the word of God which ye heard of us, ye received it not as the word of men, but as it is in truth, the word of God, which effectually worketh also in you that believe." This was what he wanted for the Galatians and why he stressed that his instructions in this letter were from God and not from man.
Note 4 at Ga 1:1: Paul's apostleship was not bestowed upon him from the earthly Jesus, as was the case with the other apostles (Mt 10:1-5). His commission was from the resurrected Lord Jesus Christ, who was raised the Son of God in power (Ro 1:4).