I've already referred to and quoted M'Crie in my article on latitudinarianism (see the end of the article for an extended quotation) with regard to his opposition to that idea (latitudinarianism being the concept that the church ought to refrain from maintaining unity and discipline within itself over Scriptural doctrines and principles that are considered "less important" than other ones). Immediately after his attack on latitudinarianism, M'Crie attacks another unscriptural method of trying to preserve the unity of the church:
Another plan of communion, apparently opposite to the former, but proceeding on the same general principle, has been zealously recommended, and in some instances reduced to practice, in the present day. According to it, the several religious parties are allowed to remain separate, and to preserve their distinct constitution and peculiarities, while a species of partial or occasional communion is established among them. This plan is liable to all the objections which lie against the former, with the addition of another that is peculiar to itself. It is inconsistent and self-contradictory. It strikes against the radical principles of the unity of the Church, and confirms schism by law: while it provides that the parties shall remain separate, at the same time that it proceeds on the supposition that there is no scriptural or conscientious ground of difference between them.
By defending such occasional conformity, English Dissenters at a former period contradicted the reasons of their dissent from the establishment, and exposed themselves to their opponents: for where communion is lawful, it will not be easy to vindicate separation from the charge of schism. The world has for some time beheld annually the spectacle of Episcopalians, Presbyterians, Independents, Methodists, and Seceders, sitting down together at the Lord's table, and then going away and maintaining communion, through the remainder of the year, on their own separate and contradictory professions. Nay, it has of late become the practice to keep, in the same church, an open communion table for Christians of different denominations on one part of the day, and a close one for those of a particular sect on the other part of the day; while the same ministers officiate, and many individuals communicate, on both these occasions. And all this is cried up as a proof of liberality, and a mind that has freed itself from the trammels of party.
It is difficult to say which of these plans is most objectionable [that is, latitudinarianism or the idea he has just been describing]. By the former, that church which is most faithful, and has made the greatest progress in reformation, must always be the loser, without having the satisfaction to think that she has conveyed any benefit to her new associates. It behoves her profession and managements to yield, and be reduced to the standard of those societies which are defective and less reformed. And thus, by a process opposite to that mentioned by the Apostle, those who have built on the foundation "gold, silver, precious stones," are the persons who shall "suffer loss" (1 Cor. 3:12, 15). By the latter, all the good effects which might be expected from warrantable and necessary separations are lost, without the compensation of a rational and effective conjunction; purity of communion is endangered; persons are encouraged to continue in connection with the most corrupt churches; and a faithful testimony against errors and abuses, with all consistent attempts to have them removed or prevented, is held up to odium and reproach, as dictated by bigotry, and as tending to revive old dissensions, and to defeat the delightful prospect of those halcyon days of peace which are anticipated under the reign of mutual forbearance and charity.
What M'Crie attacks here is that very same semi-congregationalism or denominationalism that has infiltrated so much of Reformed thinking these days. "We don't need to be united. We can just continue to exist as one, big, happy group of independent Reformed denominations, getting along nicely and working together without having to be in mutual submission to each other." The problem, as M'Crie points out, is that it "strikes against the radical principles of the unity of the Church, and confirms schism by law." If two denominations--say, the PCA and the OPC--are close enough that they can accept each other as legitimate churches, with legitimate ministries, to whom it is appropriate to recommend people for membership, etc., then they ought to be united in one denomination, for it is schism for them to continue divided. If, on the other hand, there is warrant for continued separation, they must regard each other as having something wrong with them which warrants refusing to recognize their authority by being in mutual submission to them in common councils. If the OPC, for example, regards the PCA as being in a bad enough state to warrant remaining separate from them, they ought not at the same time to be approving their ministry, recommending people to them for membership, or in general treating them like a good, legitimate, non-schismatic, Reformed church, for this attitude and practice undermines the claim to have a basis for continued separation.
I am reminded of James Durham's comments from his famous work, Concerning Scandal: "[B]y way of precept there is an absolute necessity of uniting laid upon the church, so that it falls not under debate ‘Whether a church should continue divided or united . . .' more than it falls under debate whether there should be preaching, praying, keeping of the Sabbath, or any other commanded duty." Therefore, two churches accepting each other as legitimate and as having authority must be united to avoid meriting the charge of acting schismatically.
The Free Presbyterian Church of Scotland has articulated the right attitude regarding the implications of denominational separation:
The Synod . . . desires to state that in terms of its Constitution, this Church has taken up a separate position from other Churches in Scotland in order to maintain a testimony to the infallibility and inerrancy of the Scriptures as the Word of God and in order to adhere in its practice to that Word as its supreme standard, and to the Westminster Confession of Faith, which is based upon the Scripture, as its subordinate standard. This separate position is justified because, and only as long as, it is necessary.
Accordingly, conduct giving the impression that there is no obstacle to association with other Churches undermines the necessity for a separate position and is therefore inconsistent with loyal adherence to the Free Presbyterian Church, and is consequently disapproved of by this Church.
So what does this mean for how individuals ought to relate to denominations that are wrongfully separate from the FPCS (which is to say all other denominations, for there is no just ground of separation from the FPCS, for otherwise it would not have a right itself to separate existence)? The FPCS catechism makes this clear:
146 Q. When should individual believers separate from the fellowship of others?
A. The Scriptures enjoin believers to withdraw themselves from those who are professed brethren and who walk disorderly (2 Thess. 3:6), so when men have so rejected sound doctrine, right government, and discipline, or have introduced superstitious worship, or are maintaining a schismatic position, and when an orderly correction of these evils fails, then believers are to separate from such.
There may still be informal fellowship, but there must not be the kind of formal connection which would imply that there is no reason for continued denominational separation, for, as M'Crie said, "where communion is lawful, it will not be easy to vindicate separation from the charge of schism."
For more, see here, here, and here.