We were talking about St. Paul's Letter to the Galatians at our Lay Fraternities of Saint Dominic chapter meeting last night. (For my Catholic readers, that's the one he wrote when he was stomping mad that people were saying you had to follow the Jewish Law to be Christian.)
The thought occurred that the Judaizers (as they're called) who come under St. Paul's fire and brimstone ("if anyone preaches to you a gospel other than the one that you received, let that one be accursed!") probably didn't know they were Judaizers. I expect they considered themselves orthodox Christians, fighting the good fight against old Paganizer Paul.
Jewish Christianity is what they had seen, been taught, and lived for a decade or more before all this nonsense about Gentile Christianity started being spread about. How were they to know that they were the villains of the piece? Jesus Himself told people to listen to the Pharisees, and that He had not come to abolish the Law. Who was Paul of Tarsus to say otherwise?
I take two things from this line of thought:
First, a reminder that it's a good idea to try to distinguish error from motive, and leave judging the motive to God.
Second, an awareness that anyone can mistake the incidental for the essential, if they do not keep their eyes fixed on Jesus Christ, Who gave Himself for our sins.