The problem is the whole ceremonial/moral law divide is nonsense. The OT laws are not arranged into moral and ceremonial categories, no such division could have existed in the minds of its original audience and no indication is ever given in the text as to such categories. The only way to make such categorisations is to impose totally arbitrary criteria. Admittedly Keller's criteria has the appearance of being less than arbitrary,
One way to respond to the charge of inconsistency may be to ask a counter-question—“Are you asking me to deny the very heart of my Christian beliefs?” If you are asked, “Why do you say that?” you could respond, “If I believe Jesus is the the resurrected Son of God, I can’t follow all the ‘clean laws’ of diet and practice, and I can’t offer animal sacrifices. All that would be to deny the power of Christ’s death on the cross. And so those who really believe in Christ must follow some Old Testament texts and not others.”
Now before I am misunderstood I both advocate Christological readings of the OT and am a conservative on issues of sexual immorality. Keller has ably demonstrated why some OT laws are not followed by Christians. What he has failed to do is show why any OT laws should be followed by Christians. Obviously Keller has in mind Leviticus 18:22 and 20:13 which he wants to still carry weight because to him this is not a ceremonial issue but a moral one. But the argument from the other side of the debate is that both these chapters are set firmly in the context of OT worship and avoidance of idolatry, yes the chapters feature incest and bestiality, but they also feature Canaanite idolatry and unclean animals. More to the point, "man lying with man as with a woman" is more likely a description of male temple prostitution than a modern monogamous and loving homosexual relationship. The modern social construct of homosexuality did not exist when Leviticus was written so it can hardly be expected to be addressing our contemporary situation in quite such a direct and convenient manner.
Keller is wrong because Christians do not follow any of the OT laws - we follow Christ. We may extract principles from the OT for our theology and ethics - an example of where this is done constructively is around the year of Jubilee, an example of this being less useful is Leviticus 18 - but this is fraught with danger if not done very carefully. As Christians we only read the OT as Christians, we are God's people not because we follow the law but because of the blood of Jesus (Rev 5). We read the OT to meet Christ, not to find legal precedent for our moral convictions. So where do we get our moral convictions from? Well I'll have to leave that for another post, it is time for breakfast and to wake my kids up for school.
Let me know what you think, :-)