It is clear that Christians ought not to have any fellowship with false teachers. What is less clear is what is meant by that word, 'fellowship'. In its narrowest sense it would refer to table and pulpit fellowship - that is to say that we are not to admit to the Lord's Table or to the pulpit of the Church those who do not teach according to the truth as it is in Jesus. By the same token we are not to participate in their table, or sit under their teaching. Thus a Christian cannot, and should not, participate in the Mass. To be a member of a congregation that is committed to heresy, either in its basis of faith (for example, a Unitarian congregation) or de facto (e.g. a liberal Methodist Church) is clearly wrong.
But what after that? Is it wrong for a Christian to be a member of an evangelical congregation that is part of a mixed denomination such as the URC? If so, Spurgeon was in error when he told people who had not left the Baptist Union to remain in the Union and to go on fighting to regain it. Is it wrong for a minister to accept an invitation to speak in a liberal congregation? If so I can think of a dear brother in Christ who himself left the Baptist Union after years as a BU pastor who still preaches at BU Churches. He is as valiant for truth as a man can be, even if we disagree on certain minor points of what we should be contending over. If anything he's perhaps over-zealous. Is he wrong to accept preaching invitations from such churches? Does that constitute 'Fellowship'? These are grey areas, and we need to acknowledge that they exist.
Secondly we need to acknowledge that good, sound men differ on their way of dealing with these grey areas. That's why some of us are not members of denominations we once were members of, while other good evangelical men are members of those denominations. We disagree, but we affirm that the situation is complex and there are no easy answers. Good and godly men facing the same grey areas may come to different conclusions. The fact that I think another man's approach is wrong does not mean I should denounce him in print or on the internet as a compromiser! Instead I should seek to understand why he does what he has done. A rush to judgement helps no-one and in fact damages the body of Christ. It also opens the one rushing to serious misinterpretation. To condemn someone because they have done something that someone, somewhere might possibly interpret, if they were thick as two short planks, as an endorsement of a false teacher is going too far and begins to wear the appearance of a witch-hunt.
Thirdly, we have to credit our fellow believers with some intellect. To take the recent Michael Horton example, Michael Horton is a seminary professor. His books are not exactly light reading, and are full of warnings against false teaching (my introduction to Horton as an author was through his book The Agony of Deceit). His readers are not typically ignorant, shallow evangelicals. He is best known for his work on The White Horse Inn, a programme that aims to help Christians to understand what they believe and why they believe it. It is a rather cerebral show, and Horton's followers are typically quite cerebral. In other words they are the last people on the planet to take a photograph of Horton with Rick Warren as a sign that Warren's fine and we are all to link arms with him and sing Kumbaya. Those who are likely to take such a shallow approach to a photograph are unlikely to know who Michael Horton is in the first place. Those are the people Warren appeals to, not Horton.
Thus we come to the question of 'secondary separation', as it has been called. No, in my opinion this can only meaningfully be done where one was actually united with someone in the first place. So, I am a pastor of a rather conservative Independent Evangelical Church that is a member of the FIEC, and of a local body called NoSFEC. John Piper is not a member of either of these groups, and nor is his Church. Thus, however much I may disagree with John Piper having Rick Warren speak at Desiring God, I cannot separate from Piper in any meaningful way, because I was never in fellowship with him in any meaningful way in the first place! I may voice my disapproval, but I cannot do anything else!