However, the doctrinal foundation is not enough. Before you choose specific ministry methods, you must first ask how your doctrinal beliefs "might relate to the modern world." The result of that question "therefore form[s] a theological vision." In other words, a theological vision is a vision for what you are going to do with your doctrine in a particular time and place. And what does a theological vision develop from? Lints shows that it comes, of course, from deep reflection on the Bible itself, but it also depends a great deal on what you think of the culture around you.
This concept of a theological vision explains how, for example, our conservative Presbyterian denomination, in which all churches share the same detailed doctrinal foundation (Westminster Confession of Faith) can be deeply divided over ministry expressions and methods, such as music, preaching style, approach to organization and leadership, forms of outreach and so on. The reason is that churches with the same basic doctrine are shaped by different theological visions because they are answering those questions about culture, tradition, and rationality differently. [Tim Keller, Center Church: Doing Balanced, Gospel-Centered Ministry in Your City (Grand Rapids, MI: Zondervan, 2012), 18-9]
So what is a theological vision? It is a faithful restatement of the gospel with rich implications for life, ministry, and mission in a type of culture at a moment in history. (Ibid., 19)
...a theological vision creates a bridge between doctrine and expression. It is central to how all ministry happens. Two churches can have different doctrinal frameworks and ministry expressions but the same theological vision—and they will feel like sister ministries. On the other hand, two churches can have similar doctrinal frameworks and ministry expressions but different theological visions—and they will feel distinct. (Ibid., 21)
According to Tim Keller, there is "hardware," "software" and something called "middleware" (pp 16-7), basically the middleman between "hardware" and "software." There are doctrines ("hardware") and applications ("software") and between the two is "theological vision." In other words, one starts with doctrines, and, instead of going directly to applications, one construct a "theological vision" first before applying the truths. Elsewhere, Keller explicitly utilizes the tri-perspectivalist hermeneutic to construct a part of his "theological vision."(pp. 299-301), which seems to suggest that the tri-perspectivalist hermeneutic lies behind some of the construction of what he calls a "theological vision."
The interesting thing about this category of "theological vision" is that it exists independently from the doctrinal foundation. Although Keller does state that one moves from the doctrinal foundation to the "theological vision," yet he claims that (1) the same doctrinal foundation does not necessarily lead to the same theological vision, (2) different doctrinal foundations may lead to the same theological vision. Since such is the case, "theological vision" has no necessary relationship with one's doctrinal foundations. The directionality of thought presumably is what happens temporarily, not what happens logically.
From a Reformed perspective, this new category is a big problem, since it cannot be grounded in Scripture, for otherwise it would not be possible for people with different doctrinal foundations to have the same "theological vision." It seems here that Keller's tri-perspectivalist hermeneutic has resulted in greater weightage given to the "situational" and "existential" perspectives, such that the difference in the normative perspective between two churches with differing doctrinal foundations but the same "theological vision" is reduced in significance. For if the "normative" perspective is made primary, then the differences between two churches with different doctrinal foundations cannot be reconciled even if they have similar "situational" and "existential" perspectives.
Herein lies the main problem with regards to Keller and his idea of "theological vision." Scripture speaks of its primacy in the life of the Church, and speaks as to how the Church is to conduct herself. Therefore, while one should think about the "situational" and "existential" perspectives, those should not in any way alter the primacy of the "normative" perspective, to use the language of tri-perspectivalism. Those in Reformed and Presbyterian circles, like Tim Keller, claim to hold to the RPW (Regulative Principle of Worship), therefore one does NOT have the right to innovate church practices where Scripture IS silent.
Thus, Keller's "middleware" functions like a trojan horse to smuggle in one's opinions derived from what one thinks of the culture ("situational perspectives") as having greater primacy over what Scripture commands (RPW). As we will see, Keller uses it to smuggle in all manner of stuff not found in Scripture, under the guise of "contextualization" (another problematic word).