Years ago, I was talking to the president of a parachurch Bible college, and he said (I'll never forget the exact words): "It's impossible to be consistent in matters of separation." In essence, he was telling me that no one could possibly obey Scripture. I didn't believe him, but I know why he thinks the way he does. Most people don't take Scripture seriously enough to be consistent in separation. It isn't possible without a Biblical ecclesiology, either in doctrine or practice. Seeing that he was working out of a Scripturally untenable institution, he chose just to be inconsistent. Calling disobedience inconsistency makes it sound like it's permissible. It isn't. We can see the problems that occur even short-term for us, but even greater, that God isn't pleased with its faithlessness.
I wish this wasn't such a touchy issue. Do you think it might be so sensitive because Satan is fooling a whole lot of people? I think so. He doesn't want them to get this right, because as long as He has them fooled, people won't be able to obey God's Word. I'm going to try to explain it very simply to you, and maybe touch on this in future weeks to be a greater help.
Consistency in separation is based upon the right understanding of the church, because separation itself is rooted in the church. Unity and purity are inextricably connected. We must have unity. True unity is based upon purity. This can be accomplished only in a local church. The local church (the only church) has the same doctrine and practice and the means to keep them pure. A local church can enforce one doctrine through church discipline. A local church can keep that doctrine and practice pure by fellowshipping only with churches of like faith and practice. In so doing, this church is consistent in the practice of separation.
If the church is all believers, then everyone must work together for unity. However, the differences in doctrine and practice among all believers is massive. In trying to get along, believers lose purity. In attempting to have purity, they lose unity. To be able to have both, men reduce doctrine and practice to something that they think that all the saved can agree upon, something like five fundamentals. But then, some men will say it should be more than five fundamentals. Most of these at least narrow the doctrine and practice down to essentials and non-essentials to make unity the most crucial doctrine. Making a said "non-essential" to be an "essential" might be called legalism. For some, the doctrine of unity is labeled "agreeing to disagree." In an attempt to find some kind of unity, churches or pastors will get together based on a common doctrinal statement, and then preach messages somewhat generically and have discussions that are non-controversial. Truth stops becoming the common denominator. Now unity is more important than doctrine. The worst offense is not fitting in. And it really isn't unity either, because all believers have splintered into even dozens of groups---the BBF, FBF, GARBC, SBC, etc. If not fellowships, then colleges or seminaries or very influential men become common ground. All of this, every bit, comes out of the belief that the true church is all believers, rather than the exegetical position that the true church is an assembly of baptized believers, only local.
So you see, with the wrong view of the church, both unity and purity are destroyed. As a result, right doctrine and practice are corrupted, and God isn't honored. On the other hand, with the right view of the church, God's tools of church discipline, the Lord's Table, pastoral authority, and real separation, among other things, work towards the consistent unity and purity that God designed. None of us should settle for anything less.