"Denominationalism" today dominates much of the thinking and practice of Christians, including Reformed Christians. It can be defined as the idea that it is acceptable (either permanently or for a limited period of time) for the one true church of Jesus Christ to exist in the form of multiple divided denominations.
Denominationalism is contrary to the nature of presbyterian church government. In a presbyterian system, all the churches of Christ have an obligation to be in formal communion with each other under mutually-binding councils. Local congregations are to be overseen by a body of elders (typically referred to as a "session" or a "kirk-session"). Individual sessions are to be united together in common presbyteries, which (typically) are councils made up of representatives of all the sessions in a given region. Presbyteries are to be united in yet higher common councils. This pattern is to continue until we reach the highest council, the ecumenical or general council of the whole church, which is over all the churches of Christ in the world. No de jure true churches are to be left out of this interconnected web of churches. (See here for a classic description of this form of church government.)
Denominationalism violates presbyterian church government by endorsing a situation where there are multiple acknowledged true churches which accept no obligation to be united to each other in a presbyterian system. Many Reformed people today seem to advocate what I call a semi-congregationalist form of church government, which insists that local congregations must be united together in higher councils in a presbyterian manner but that this process can stop at some arbitrary point before it includes all the churches of Christ in the world, leaving "clumps" of independent denominational churches. One of my goals for this site is to raise awareness of this deviation from presbyterianism and encourage Reformed and other Christians to embrace a fully consistent presbyterianism.
In a presbyterian system, when denominations are divided from each other, the implication is that these denominations are rejecting each others' de jure legitimacy and authority. They may accept each others' de facto existence as churches, but they do not formally recognize each other as having legitimate authority. If they did recognize each other, they would unite in mutually-binding councils and thus be in full communion with each other. Therefore, if we take the authority of the church seriously (in accordance with Westminster Confession of Faith 31:3), we must be concerned to be united to the proper denomination. We cannot just choose any "sufficiently" orthodox denomination we come across or happen to like. If two denominations have rejected each others' authority, either they are both wrong to have done so, or one of them is right and the other is wrong. There are valid and invalid reasons to refuse to unite. If one of those denominations is correct in refusing to unite with certain other denominations, proper respect for the authority of de jure church courts requires us to abstain from union with the rejected denominations (as much as possible, all other things being equal) and to unite with the proper denomination.
I would like to put forward the Free Presbyterian Church of Scotland as being the proper denomination to be united with. An outline of my reasons for this claim can be found here. A major part of my reasoning, of course, is my set of doctrinal and practical convictions as a historic Presbyterian with full subscription to the original Westminster Confession of Faith (as well as the Larger and Shorter Catechisms, and other documents of the Westminster Standards). I do not think that the FPCS is perfect or that it is in no need of reform, but I think that its claim to have a right of separate existence from other denominations is justified. This is my conviction from my evaluation of the evidence.
The other main purpose for this page, then, is that I would like to invite all Christians (and particularly Reformed Christians) to investigate the claims of the Free Presbyterian Church of Scotland and to consider helping to end the proliferation of denominations in the world by joining her. I encourage all people to consider whether or not the denominations they are currently members of have a valid claim to separate existence from the FPCS. If they do not, then the next moral duty, inasmuch as it is practical to do so, is to begin a process of coming into full communion with the FPCS.
This page can be used for dialogue and debate relating to my claims about the FPCS and about denominationalism and presbyterian church government. Please feel free to post questions, comments, arguments, etc., relating to these issues, and we can try to establish a productive and respectful dialogue. My ultimate goal in this dialogue is to convince others to abandon denominationalism and to join the FPCS. If it turns out I am wrong about any of these things, then you are welcome to try to show me where I have gone wrong and to set me straight. All I ask is that the conversations are conducted efficiently, productively, and respectfully. If you are a member of the FPCS, of course I welcome and invite your input as well, and I will try to solicit involvement from others who are favorable towards the FPCS.
For more on the various ideas expressed above, see here and here, and in general here.
UPDATE 7/16/13: Some great reading on the subject of the biblical doctrine of the unity of the church can be found here, here, and here.
UPDATE 8/13/13: I should add a brief note here to address something that wasn't articulated in the comments above:
I do not believe it is ideal that Christians in countries outside of Scotland should be members of the Free Presbyterian Church of Scotland. Ideally, the Church of Scotland ought to be limited to Scotland. The current situation, however, is that the FPCS is not in formal communion with any other denomination around the world. It remains separate from them all, and, assuming it is right in retaining this separation, this leads to a moral duty (all other things being equal) for all of us to seek as much as possible to be in communion with the FPCS and avoid communion with denominations that are not in communion with it, for reasons described above.
However, what we would like to see ultimately is each nation having its own orthodox national church in full communion with the FPCS, united under a binding, presbyterian international council. For now, for example, it is right for there to be an FPCS in Santa Fe, TX; but ultimately, we would want to see enough churches in the USA to establish a national body that is no longer under Scottish oversight (while still being in full communion with and mutually accountable to the Scottish church). And it is not hard to see that the more we all take these issues seriously enough to act accordingly, even to the point of joining the FPCS, the closer we will be to being able to reach this long-distance goal of having indigenous national churches in full communion with each other and the FPCS. On the other hand, if we all wait until there is already such a national church in our own land before we leave the schismatic denominations and unite with the FPCS, how will such a church ever be built up? It may be the case that some particular denomination in our own land will reform itself to the point that it can join in full communion with the FPCS and become a proper national church, but we cannot wait for this to happen before we consider what our own personal duties may be in these matters.