From a quick reading of all 6 chapters, one can tell that, apparently, the church of Galatia was having a difficult time with sticking with the Gospel. False teachers have moved in after Paul and are preaching a works-oriented salvation. Paul is writing this letter to refute that, plain and simple.
Paul, an apostle, (not of men, neither by man, but by Jesus Christ, and God the Father, who raised him from the dead;) and all the brethren which are with me, unto the churches of Galatia: grace be to you and peace from God the Father, and from our Lord Jesus Christ, who gave himself for our sins, that he might deliver us from this present evil world, according to the will of God and our Father: to whom be glory for ever and ever. Amen.
I marvel that ye are so soon removed from him that called you into the grace of Christ unto another gospel: which is not another; but there be some that trouble you, and would pervert the gospel of Christ. But though we, or an angel from heaven, preach any other gospel unto you than that which we have preached unto you, let him be accursed.
In the first 5 verses Paul spells out the foundation for his apostleship (Christ) and gives the core of the Gospel (who gave himself for our sins, that he might deliver us from this present world.) Verse 6 begins his reprimand.
He is amazed that this church has so quickly been swayed in what they have called truth. They have so quickly claimed a new shepherd and forsaken the grace of God. People ask about the two uses of 'another,' one in verse 6 and one in verse 7. They are two different Greek words translated into the same English word. The first use of another carries the idea of being something completely different while the second carries the idea of a different presentation of the same truth. Paul is basically saying, "You have chosen to follow a completely different Gospel, not a different presentation of the true Gospel but a different Gospel altogether! A perverted Gospel."
In verse 8, he levels his charge. He instructs these wayward believers that unless an angel, the brethren with him or he, himself gives them a different Gospel, that they should not believe it. Anyone that preaches a different Gospel should be anathema.
People might interpret this attitude as being one of haughtiness, a 'holier than thou' position. Not so. Paul knew he had the truth. There was not a single doubt in his mind at all. He was passionate about the truth and was willing to take a bold and daring stand for it. In the next few verses you see why, but for right now, I leave you with this challenge.
Every day, the Gospel we know is challenged. We are faced with compromise. Asked to water down the cross, to magnify the grace, to downplay the holiness and to steal the glory. There is only one truth.
Are you standing for it?