The Law Firm of the (DA) Carson and (Tim) Keller on contextualization:
“The fact that we must express universal truth in a particular cultural context does not mean that truth itself is somehow lost or less universal. D.A. Carson writes, ‘[While] no truth which human beings may articulate can ever be articulated in a culture-transcending way… that does not mean that the truth thus articulated does not transcend culture.'” (Center Church, 93)
What Keller and Carson are saying here is that, on the one hand, it is impossible to communicate the gospel in a way that somehow transcends culture. As soon as you express the gospel you are doing so in a way that more accessible to some and less accessible to others. You do this through language, use of metaphor, length of presentation, etc.
On the other hand, don’t fall into the trap of many liberal theologians today and make the assumption that because we always communicate to a particular culture, that the truth itself is culturally bound. The gospel is not the product of culture, it transcends and judges culture. The gospel is universally true.
The task of contextualization, then, is to communicate the universal, culture-transcending truth of the gospel in a particular way to make it accessible and understandable to the hearers.