Yes, your quote from Galatians is often used as well by those who urge caution re: Marian apparitions; however, you might check out the following from Luke 9:49-50 in regard to tolerance of those who  preach the gospel of Jesus, are self proclaimed Christians, but not Catholic-Christians. 
"It was John who said, "Master, we saw a man using your name to expel demons, and we tried to stop him because he is not of our company." Jesus told him in reply, "Do not stop him, for anyone who is not against you is on your side."