Another aspect of this contrast community is unity across church communities and denominations. In Christendom, when "everyone was a Christian," it was perhaps useful for a church to define itself primarily in contrast with other churches. Today, however, it is much more illuminating and helpful for a church to define itself in relationship to the values of the secular culture. It we spend our time bashing and criticizing other kinds of churches, we simply play into the common defeater that all Christians are intolerant. While it is right to align ourselves with denominations that share many of our distinctives, at the local level we should cooperate with, reach out to, and support the other congregations and ministries in our local area. To do so will raise many thorny issues, of course, but our bent should be in the direction of cooperation. [Tim Keller, Center Church: Doing Balanced, Gospel-Centered Ministry in Your City (Grand Rapids, MI: Zondervan, 2012), 260]
Openness to cooperation is another essential movement dynamic. Because members of the movement are deeply concerned with seeing the vision accomplished, they are willing to work with people who are materially committed to the vision and share primary beliefs but who differ in preferences, temperaments, and secondary beliefs or are members of other organizations. ... In the Christian world, this means Christian groups with movement dynamics are more willing to work across denominational and organizational lines to achieve common goals. (Ibid., 349)
Evangelicalism (the original one) is a movement that arose out of the 18th century First Great Awakening. Prior to that, reform was done through the church, and while in places like England something like different "denominations" eventually came to co-exist, for the most part each nation had one national church which was supposed to function as the church for the people in the vicinity. Even in England, the Church of England was (and currently still is) the national church and everyone in the districts were to go to their parish churches. Dissenter congregations were tolerated only out of necessity, not out of a desire to actually have multiple denominations. The original Evangelicalism desired only to focus on the Gospel and cooperate across the various denominations which had arisen by the 18th century. While not necessarily denying the importance of other doctrines of the Christian faith, the differences were reduced in importance as the focus was on proclaiming the Gospel unto the personal conversion of the masses.
Keller is an Evangelical (big "E"), which is to say he promotes cooperation across denominational lines just like the original Evangelicals do. On top of the usual reasons given for "cooperation," Keller added the reason that we should not "play into the common defeater that all Christians are intolerant." But is the Evangelical rationale and manner of cooperation across denominations valid?
First of all, we admit that all who profess the true Gospel, regardless of denominational affiliation, are our fellow brothers and sisters in Christ. But that does not mean that one treats churches the way one treats individual believers. As an extreme counter-example, just because one person who is in the Mormon temple actually believes the true Gospel does not mean that we should start treating Mormonism as Christian. Rather, we exhort the person to leave his false religion and identify with a true Christian church. Of course this is an extreme counter-example, but it serves to illustrate the principle that just because there are true Christians in a religious group does not necessarily imply that one treats the group as fully Christian.
With regards to church history, Keller is in error concerning why the different denominations exist. Besides those from different national backgrounds, those that exist in the same country are different denominations because they are real doctrinal differences separating them. For example, English Presbyterians were separate from the Church of England because Presbyterians believed in Presbyterian polity and were against Romish elements in worship, as seen in vestments and mandated liturgies. Baptists are congregationalist in polity and deny infant baptism. The separation between them was not due to any supposed competition for market share as Keller implied, but rather because each side believed their distinctive doctrines to be biblical and important; it had nothing to do with the existence of Christendom! Keller is also naive if he thinks that "cooperation" across denominations would stop unbelievers from thinking Christians are intolerant. They think Christians are intolerant because Christianity is exclusive, not because Christian denominations define what they believe in contrast to other denominations. In the same vein, Keller is in error when he claims that we should define ourselves against the secular culture as opposed to against other churches. First of all, we should not define ourselves primarily against anything. We define ourselves as Christians who believe certain doctrines that we hold to be biblical, and THEN we define ourselves against those who disagree with us. We shouldn't even define ourselves "against the secular culture," for not everything in the secular culture is wrong!
Keller speaks about "movement dynamics," which is congruent with his Evangelicalism because Evangelicalism is after all a movement not a church. And if one actually follows Keller's Evangelicalism, then Keller is absolutely right about such cooperation. But why should we be Evangelicals (big "E")? Now, no doubt there is nothing wrong with individual believers coming together for things like prayer, but we are speaking about the churches here. If churches are churches and not movements, then we should question whether such cooperation is what churches ought to do.
The fact of the matter is that churches, if they are to be churches, hold fast to what they consider to be the biblical pattern of sounds words, which in Presbyterianism is the Westminster Standards. Presbyterian churches are not "Gospel-only" movements (which is what Evangelicalism is), but churches confessing the pattern of sound words that the Church believes. As such, cooperation is to be done only between churches of like faith professing the same pattern of sound words. In America, such cooperation and unity is expressed through NAPARC (North American Presbyterian and Reformed Council).
Keller's idea of cooperation across denominations fails to understand the true nature and responsibility of the Church. The Church is to stand for the pattern of sound words she believes to be biblical, and cooperation is extended to all who holds to the same vision. His Evangelical Gospel-Onlyism results in him supporting even Pentecostal churches despite the fact that such churches are sub-biblical in life and doctrine. Against, just because there are many believers in Pentecostal churches does not have any bearing on whether the church they are in should be considered a true biblical church.
True biblical cooperation is to be done with churches that share a similar confession of biblical doctrine, which for whatever reason are unable to unite into a single church body. As I have said many times before, the only legitimate reason for having different denominations is doctrinal. Practical issues might prevent union between two church bodies with a similar confession, but such separation should be seen as a necessary evil with hopes of one day being rectified.
Reformed bodies are separate from non-Reformed bodies due to confessional differences. But those like Keller have no excuse for being separate. If Keller believes that such differences are insignificant such that he can help Pentecostal churches, then why isn't he in ecumenical talks with those churches? Separation without cause is schism, a breaking of the unity in Christ. While the Reformed have a legitimate reason for remaining separate, Evangelicals like Keller have no reason why they aren't actually united with other Evangelical-minded churches. If other doctrines are insignificant for ministry, then they should not divide those same churches from each other. Keller, if he were to be consistent with his Gospel-Onlyism, should just leave the PCA and form an "Evangelical" denomination or association consisting of churches like the Pentecostal and Baptist churches he had helped.
The ironic thing about such united Evangelical projects is that they missed out on true Christian unity. Those who proclaim the same understanding of the faith are united, and not just on the surface-level. Even more ironic, there is a deeper unity between confessional Presbyterians and confessional Reformed Baptists that will forever elude those surface level "Evangelical" cooperation. The funny thing is that true spiritual unity comes when one is not seeking it but seeking Christ and His truth, while for the Gospel-Only Evangelicals who seek such unity, true spiritual unity will forever remain a mirage.