2 Cor. 6:14-18 gives us the explicit, unambiguous, and clear command to separate ourselves from the religious elites, the Nicolaitans who run the "churches," and those in 'para-church' organizations that teach lies instead of the Truth of God's Word. This includes EVERY nationally/internationally known "ministry," all of their 'off-shoot' organizations, their special gatherings (crusades), their books, their blogs, their defenders, their songs, and all the other tactics used to deceive the lost and/or ignorant followers. Be Ye Separate! It is self deception to remain in any of these groups with the thought that your presence will "be a witness" to them and they will "see the light" if you continue with them long enough. Not only is it self deception, but in direct opposition to the commands of the Word. Titus 3:10 speaks plainly: "A man that is an heretic after the first and second admonition reject." It says "reject," not coddle, baby, treat with kid gloves, or walk around on egg shells---------reject. Matt. 10:14; Mark 6:11; Luke 9:5 and 10:10 & 11 tells us the same thing, if a city or a house or a person will not receive the Word, then we must shake off the dust on our feet as a testimony AGAINST them. Too many have the mind set of Lot, who chose to stay in the cesspool rather than to separate himself from the obvious evil that engulfed him. But God had mercy on him and forcibly removed Lot in order to destroy Sodom and the surrounding cities.
Would we sit on our ego enshrined pedestal thinking that we may be responsible for "converting" an "errant brother" (who is likely no brother at all) by our continued submission to his lies? It is submission by the very fact that we refuse to remove ourselves from the cesspool created to deceive. 2 John 8-11 provides further injunction against the notion that we can convert such to the truth. Not only are we not capable, we are intruding upon the work of Another, the Holy Spirit, whose task it is to convict men of sin, of righteousness, and of judgment. Will we have our ego-infested mind set go so far as think ourselves indispensable to His work and that He can't do anything without us? [This has been taught for decades in the American "church" by innuendo with the required "reading between the lines" nonsense.] This "staying as a witness" (ego trip) has further ramifications for those who would participate in it's insanity: such a person is found to be in direct opposition to the Lord he claims to love and serve and thus "shares" in the evil deeds of the one he seeks to convert---2 John 10 & 11.
Some may raise James 5:19 & 20 in objection. But there is no conflict in Scripture, nor is there here with the teaching of separation. Note that in James the person admonished receives the correction, repents, and returns to his Lord. Not so the case of those spoken of in the other verses cited. Separation from the evil teachers and supporters of heresy is a must for every believer. Dozens of examples can be found in the Old Testament, especially as Israel "dispossessed" the inhabitants of the Promised Land and took possession of it for themselves.
Today, we see the Lord working with His elect much the same as He did Lot centuries ago. Just as Lot did not want to leave his comfortable cesspool (the attraction he had for Sodom is not stated) and was even willing to sacrifice his daughters to the evil men outside his door, it was God who intervened and extracted Lot and his family from this mecca of sin. How many of us today have faced evils in our "church" and refused to leave even in the knowledge that rank heresy was being taught? Yet the Lord, in His everlasting mercy, brings circumstances to bear upon us and quite literally forces His elect out of the idol infested buildings that call themselves a "church." It may be, no will be, painful. Those that were considered 'friends' will ignore you, even shun you, speak evil of you, try to lay guilt at your feet, or try to cajole you back into their ranks. The greatest temptation you may face will be to just 'throw in the towel' and go back to the way things were. Remember Lot's wife.
One other 'reason' to remain in a "church" that is given is that it is for the children's sake, the programs for youth, the activities, the friendships, etc. How soon do we forget that Lot OFFERED his daughters to those evil men outside his door. Would you offer your kids to the devil so that they could have a few friends? What's the difference between staying in an heretical "church" 'for the kid's sake' and sending your kids through the fire in sacrifice to Molech? None. The arguments to stay are plentiful, guilt trips abound, emotional pain great. But once the decision is made to separate yourself and your family the overwhelming peace that passes all understanding will flood your heart and mind. It seems the scenario is always the same: doctrinal controversy, attempts to correct the evil go unheeded (usually vehemently opposed), threats from the hierarchy, emotional stress, much prayer and seeking of the Lord (He does like to walk with His children), temptation to capitulate to the demands of "church leaders," and lastly and without exception the full knowledge that the entire episode was orchestrated by your loving Father, that He is pleased with the outcome, and there really is "joy in the morning." Don't fear separation, but know that it is your Father's good pleasure to give you the kingdom---Luke 12:31 & 32.