The book, just as Acts, must be viewed through the dispensational lens, or it will not make sense and will contradict.
I use the word dispensational, because, we must see the book for what it is; an argument that we are no longer under the dispensation of the law, and to insist so is to pervert the gospel of Grace that began with Paul, therefore, he is uniquely our apostle.
It is obvious that it is impossible to argue that “by grace through faith, not of works” has always been the pathway to salvation. To make such an argument is espousing covenant theology.
Verse 1: Paul, an apostle, (not of men, neither by man, but by Jesus Christ, and God the Father, who raised him from the dead;) Paul began all his letters with his name. Again, why there is a lot of disagreement regarding Hebrews.
Notice that he says that he is an apostle not of men, neither by man, but by Jesus Christ. Obviously, he would not have said an apostle not not of men, neither by man if there were not some. Those would be men like Matthias, Barnabas (Acts 14:14), and James (Galatians 1:19). Neither of these men were numbered with the Twelve or called directly by Jesus during his earthly ministry. However, Paul's calling was unique in that he was called by God to this position without man being involved (Romans 11:13).
Why was Paul's calling so late in the game? Because Paul's ministry was to be unique in that he was not going to go to the nation as the others did, but to the Gentiles. He was called in Acts 9 by Jesus Christ, and God the Father while on the road to Damascus.
Interestingly, Paul's calling came from heaven which is the Body of Christ's destination! His argument is that he was called just like the Twelve.
Verse 2: And all the brethren which are with me, unto the churches of Galatia: We do not know who all of the brethren where with him, but we do know that Barsabas and Silas were (Acts 15:22).
Notice that he is writing to the churches of Galatia. Again, these are the churches that he had established on his first missionary journey that he refers to in Acts 16:6, Acts 18:23, and 1 Corinthians 16:1.
Notice the word churches. This is the only epistle written to multiple churches rather than a single church.
Verse 3: Grace be to you and peace from God the Father, and from our Lord Jesus Christ, this phrase is the beginning of all Paul's epistles and another argument against him being the author of Hebrews.
Notice grace and peace which was uniquely Paul's. On the other hand, the message of the others was repent and be baptized. Things that are similar are not the same.