When some of the disciples came to Jesus and told him of people doing things in His name, he replied "If they are not against us they are for us".  Is this as simple as it sounds?  I believe that in the earliest days of the Church it was easier to know which side people were on.  That is, for those who knew the truth.  There were counterfeits who fooled some of the people, like Simon Magnus. But even as convincing as he was to the average person he was exposed quickly by a true apostle of Christ. If the apostles had not come along it is very likely Simon would have continued his deception and many people would have been deceived to their destruction.  There were many descriptions given for deceivers and false prophets and it was prophesied that in the last days some would be so deceitful that they would almost deceive the "very elect".  That is a very sobering, eve disturbing thought.  Fortunately, we are given some very powerful advice - we are to "test the spirits".  Can we tell today who is against us and who is for us?