Albert Barnes on Galatians 5:8

This persuasion. This belief that it is necessary to obey the laws of Moses, and to intermingle the observance of the Jewish rites with the belief of the Christian doctrines in order to be saved.

Not of him that calleth you. That is, of God, who had called them into his kingdom. That it refers to God, and not to Paul, is plain. They knew well enough that Paul had not persuaded them to it, and it was important now to show them that it could not be traced to God, though they who taught it pretended to be commissioned by him.