The One Gospel of Christ

The claims of the Judaizers involve various doctrinal errors which Paul takes up:
The relationship between law and grace....
The spiritual nature of the promises made to Abraham....
An accurate definition of Abraham's seed who are heirs of the promise.
And a true understanding of the place of the law in the OT economy.
Then an application of that understanding to the gentiles in light of Scripture.

The entire issue in the Galatian churches boils down to the matter of truth vs. error:
Whenever gospel preaching comes to men this is the question: Men are to seek, you are to seek and encounter with God in his word. Nothing else can substitute for this in the life of the sinner. If you hear God speaking, and respond in faith, you will be saved. Otherwise, looking for something else you'll be diverted & ultimately perverted.

• Paul points this out by making reference to a hypothetical ‘Angel from heaven'...
  = This is Paul's ‘wild emphatic' way of emphasizing the point.
  = Incidently he does the same with the question of eloquence w/ 1Cor 13:1.
  = This is to say, he grasps an impossibility in order to emphasize the point.
  = It doesn't matter who is speaking, preaching a false gospel is a damnable offence.
  = Not even an Angel from heaven is immune from this condemnation.