A common objection against the practice of separation from brothers in Christ is that separation implies the brother is in sin and no true Christian can remain in sin. If the fundamentalist (here defined as an orthodox Christian who practices the doctrines of separation from both false teachers and persistently disobedient brothers both within and beyond the local church) grants this objection, he is forced either to concede that his evangelical brothers’ failure to practice separation is not sinful or he is forced to conclude that evangelicals are, in fact, not truly brothers at all.

While this objection must be (and can be) met on exegetical and theological grounds, parallel situations in church history often helpfully shed light on present debates.

An example of this may be found in the discussions about unification between the northern and southern Presbyterian churches after the Civil War. In these discussions B. B. Warfield recognized both that different sins required differing levels of responses and that in certain situations a sin may require ecclesiastical separation without casting doubt on the professed salvation of those separated from.

Note this letter from Warfield to a fellow Presbyterian pastor in 1887:

I must confess to you that I am one of those whom you perhaps consider grossly inconsistent, who heartily accord with both the deliverances of 1818 & 1845. I do think slavery a gigantic evil & entirely inconsistent with the spirit of the Gospel & a sin in the slave holders: & I do not think it a disciplinable offense or a fit test of communion. It is possible ‘to sin against Christ’ & yet not be subject to exclusion from his table (1 Cor. viii. 12, compared with the context & the parallel in Romans xiv, e.g. Ro xiv 3). . . .
. . . That the Southern Church has not repented of its sin in regard to slavery would be no bar to my union with it: I could unite with it in a free conscience tomorrow. But that it is not awake to its duty to the Freedman & that organic union with it would injure if not destroy our work among them makes me deprecate & pray against reunion in any near future.

Cited in Bradley J. Gundlach, “Warfield, Biblical Authority, and Jim Crow,” in B. B. Warfield: Essays on His Life and Thought, ed. Gary L. W. Johnson (P&R, 2007), 163.

In other words, Warfield insisted on remaining ecclesiastically separate from R. L. Dabney, but he was not casting doubt on the intransigent Dabney’s regeneration.