It is not uncommon in the circles I travel in to see very rigid boundaries marked out as to what constitutes orthodoxy, those outside of the walls being excluded from fellowship with those inside. This sort of exclusivity that forces everyone else to think within your predetermined categories is not only absurd, it is sinful. Bavinck, from the fourth volume of his Reformed Dogmatics:

It [cannot] be denied that the endless divisions of the confessors of Christ offer the world an occasion for pleasure and scorn and give it reason for its nonbelief in the One sent by the Father, inasmuch as it does not see the unity of believers (John 17:21). As Christians we cannot humble ourselves deeply enough over the schisms and discord that have existed all through the centuries in the church of Christ. It is a sin against God, in conflict with Christ's [high-priestly] prayer [for unity], and caused by the darkness of our minds and the lovelessness of our hearts.

Given this, he writes, it is understandable that many Christians would make fervent attempts to unite the Church. However, this is then usually accomplished through violent means, or by some sort of syncretism and fusion. Ultimately, the power to bring the Church together as one does not reside with us.

The failure of all those attempts has something to teach us. History, like nature, is a work of God; it does not take shape apart from his providence. Christ, by his resurrection and ascension, became king at the right hand of God and will remain king until he has put all his enemies under his feet [1 Cor. 15:25]. He reigns also over the divisions and schisms of his church on earth. And his prayer for unity was not born of unfamiliarity with its history nor from his inability to govern it. In and through the discord and dissension, that prayer is daily heard and is led to its complete fulfillment. The profound spiritual sense in which the unity of his disciples was understood by Jesus necessarily excludes all violent and artificial attempts to introduce it. Christ, who prayed for it, is also the One—and he alone—who can bring it about. His prayer is the guarantee that it already exists in him and that in due time, accomplished by him, it will also be manifest in believers.

But while we know this to be true, it certainly does not give us an excuse to neglect seeking unity now. Our task is not to erect barriers, but to tear them down. Faithfulness to Christ requires it.