Keller outlines how a knowledge of secular philosophical trends in moral standards can help create bridges to present biblical standards.
It reminds me of the Apostle Paul’s apologetic in at the Areopagus in Acts 17:22ff.
I don’t want to try and pick apart Keller’s tight light of reasoning, but would rather you went and read it yourself.
In summary, he writes:
I think that in our contemporary society, Christians’ beliefs about sex and gender will be one of the biggest points of conflict with our culture. We will need to co-opt some of our culture’s own baseline narratives (the importance of human dignity and community) in order to gain any hearing at all for our beliefs.
Read: Talking To The World at Redeemer Blogs.
mgpcpastor's blog 
I like doing what Keller advocates as well. Some don’t like it at all. I’ve been accused of not believing the Bible to be sufficient.
I was struck by the fact Keller didn’t reference Acts 17, when it seemed such a clear example of what he described.