Tim Keller, responding to charges that Christians randomly pick and choose which parts of the Old Testament laws to observe:
It is not that I expect everyone to have the capability of understanding that the whole Bible is about Jesus and God’s plan to redeem his people, but I vainly hope that one day someone will access their common sense (or at least talk to an informed theological advisor) before leveling the charge of inconsistency….
…The New Testament gives us further guidance about how to read the Old Testament. Paul makes it clear in places like Romans 13:8ff that the apostles understood the Old Testament moral law to still be binding on us. In short, the coming of Christ changed how we worship but not how we live. The moral law is an outline of God’s own character—his integrity, love, and faithfulness. And so all the Old Testament says about loving our neighbor, caring for the poor, generosity with our possessions, social relationships, and commitment to our family is still in force. The New Testament continues to forbid killing or committing adultery, and all the sex ethic of the Old Testament is re-stated throughout the New Testament (Matthew 5:27-30; 1 Corinthians 6:9-20; 1 Timothy 1:8-11.) If the New Testament has reaffirmed a commandment, then it is still in force for us today….
…Once you grant the main premise of the Bible—about the surpassing significance of Christ and his salvation—then all the various parts of the Bible make sense. Because of Christ, the ceremonial law is repealed. Because of Christ the church is no longer a nation-state imposing civil penalties. It all falls into place. However, if you reject the idea of Christ as Son of God and Savior, then, of course, the Bible is at best a mish-mash containing some inspiration and wisdom, but most of it would have to be rejected as foolish or erroneous.
So where does this leave us? There are only two possibilities. If Christ is God, then this way of reading the Bible makes sense and is perfectly consistent with its premise. The other possibility is that you reject Christianity’s basic thesis—you don’t believe Jesus was the resurrected Son of God—and then the Bible is no sure guide for you about much of anything. But the one thing you can’t really say in fairness is that Christians are being inconsistent with their beliefs to accept the moral statements in the Old Testament while not practicing other ones.