Michael Horton doesn't agree totally with Keller's description of the "Two Kingdom's" view so that also needs careful persual read it here.
A must read is the article in Themelios by Dan Strange which is very thought provoking about these issues. However it is a footnote in this article by Strange that I want reflect upon in terms of considering a person's assumptions in the arguments the Culture itself presents as cogent, reasonable and "convincing".
Consider Stranges footnote #72
As Julian Rivers pointed out in 2004, ‘It may be that a culture deviates in some respect from the law of God to such an extent that some moral positions seem defensible to Scripture alone. We may rapidly be reaching that point in the Western world as regards sexual ethics’ (‘Public Reason’, Whitefield Briefing 9:1 [May 2004]: 4). One thinks here of a country like Switzerland currently discussing the decriminalization of consensual incest and the U.S. case of David Epstein, charged with having a three-year affair with his adult daughter. Epstein’s lawyer said to ABCNews, ‘Academically, we are obviously all morally opposed to incest and rightfully so. At the same time, there is an argument to be made in the Swiss case to let go what goes on privately in bedrooms. It’s OK for homosexuals to do whatever they want in their own home. . . . How is this so different? We have to figure out why some behavior is tolerated and some is not’(http://www.huffingtonpost.com/2010/12/15/david-epsteins-lawyer-we-_n_797138.html).
To which I hope that we’d point out the assumption behind the thinking that ‘what is carried out in private is of no concern to anybody else’ is “that what I do in private doesn’t affect anybody else”. But this ignores how such activities affect one’s character and relationships in the public sphere. One cannot divorce their worldview that allows and condones such behavior from how they relate the rest of the time in public.
Just something to ponder.