Paul is teaching to Christians. what verse says 'Even from other so called Christians'?In 2 Cor 11 Paul describes some of the sufferings of others as he recounts the sufferings he has been enduring for Christ. He says to them...
19 For you gladly put up with fools, being wise yourselves! 20 For you put up with it when someone makes slaves of you, or preys upon you, or takes advantage of you, or puts on airs, or gives you a slap in the face.
These are regularly sufferings for Christians in many arenas. Even from other so called Christians.
what is a 'so called Christian' to a catholic?
there are many people who do claim to be Christian but they know almost nothing about Him, nor do their hearts desire to know about Him.