How can a city's churches become unified enough to be a movement of the gospel, even a movement of movements? They need to be part of a citywide movement of churches and ministries that exist in a supportive, mutually stimulating relationship. The assumption behind this idea is that no one kind of church — no one church model or theological tradition — can reach an entire city. Reaching a city requires a willingness to work with other churches, even churches that hold to different beliefs and practices — a view sometimes called "catholicity." [Tim Keller, Center Church: Doing Balanced, Gospel-Centered Ministry in Your City (Grand Rapids, MI: Zondervan, 2012), 368]
"I’ve learned several things in all my years being a pastor, and one of them is this: it takes all kinds of churches to reach all kinds of people." -Rick Warren (Source)
If anyone were to suspect a link between Tim Keller and pragmatists like Rick Warren, it would not be from want of evidence. Even if there is no actual relation between Keller's and Warren's ideas, the eerie similarities between the two should make anyone uneasy. But perhaps we shouldn't be too surprised since both see themselves as 'Evangelicals' and both cater to the questions and mood of unbelievers, the difference between "seeker-sensibility" and "seeker-sensitivity" being only a willingness in the former to offend with the biblical answer.
In discussing the idea of "movements," both Keller and Warren as 'Evangelicals' have came up with the exact same answer: that one needs all kinds of churches to reach all manner of people. To ensure that we did not misunderstand him as saying we need different churches for people speaking different languages, Keller mentions that one "church model or theological tradition" is insufficient to reach people with the Gospel. Evidently for Keller, God must use sub-biblical church models and theological falsehood to lure people to the Christian faith, otherwise we simply cannot reach them. One wonders where Keller's professed Calvinism has disappeared to when he made those statements.
To say that Keller's statements here are wrong is a severe understatement. If one takes it to its logical conclusion, it must mean that God is unable to save some people without using some degree of falsehood. That opens up Pandora's Box of errors in soteriology and theology proper. Needless to say, I am happy that Keller is not consistent with what he wrote. If that statement stems from his tri-perspectivalism and contextualization, then that is another reason why we should reject those two as errors.
Should there be some variety in church practice? I do not see why there shouldn't be variety concerning circumstances of worship and evangelism. Since salvation is of the Lord however, we should state that a church with one church model and one theological tradition CAN reach an entire city. Such an endeavor might need church plants using different languages, meeting at different locations, and with different service times, but it would not require changing the church's model and her theology at all.
P.S.: "Catholicity" means in line with the witness of the entire Christian church, NOT in line with those proclaiming to be churches but are either false churches or severely deficient churches. For example, "Catholicity" never meant embracing Arians or Socinians because those sects are not Christians. It also never meant embracing the Donatists even though the Donatists, for the most part, do not seem to embrace any serious heresy.