What were the Judaizers teaching the Galatians about circumcision and the Law of Moses?
The Judaizers taught that faith in Paul's Gospel was insufficient for salvation; they insisted that Gentiles must also practice circumcision and keep the Law of Moses in order to be saved, thereby corrupting the truth by mixing law-keeping with faith.
Les explains that the Judaizers were corrupting Paul's Gospel by adding requirements. Paul's Gospel says you are saved by faith plus nothing. But the Judaizers came in saying you cannot be saved that way; you have to also practice circumcision and keep the Law of Moses, or you cannot be saved. Les calls this a pollution and a corruption of the truth, like mixing water with wine. Paul wrote the whole letter to the Galatians to correct them from this error of believing something that was corrupt and perverted.