Over the next few posts I want to focus on the benefical uses of the law in the life of the believer. It has been rightly pointed out that the law of love or the ethic of love takes center place in the New Testament. I would argue that love, in fact, stood at the center of the Old Testament ethic and Mosaic Law as well, though this truth had been obscured by legalistic adherence to the external demands of the law to the neglect of the internal. Commands to love God and love neighbor appear in the Old Testament (Deut 6:5 & Lev 19:18) and are evident in the two table structure of the Ten Commandments.
That gives us some insight into one function of the law, specifically the Ten Commandments, in the life of the believer. We are called to love, but the Bible doesn't leave it to us to determine all that this love entails. The moral law explicates the law of love. The moral law makes it clear that one cannot honestly claim to love God while worshipping idols, or to love ones neighbor while stealing from him. The New Testament gets us behind the externals of the law and makes it clear that we cannot claim to love our neighbor even if we do 'loving things' while harboring hatred in our hearts (though the Old Testament also spoke to the internals as well - see the 10th commandment for a good example).
So, to love God we must obey the commandments; however, we must not assume that obeying the commandments is the sum total of what it means to love God, or our neighbor. Love for God and Love for Neighbor must be lived out, but it would be a tragic mistake to think that living out love by doing loving things is enough. Love must flow from a heart that is warm towards God and Neighbor. The balance between legalism (focusing on doing the loving things without the affections) and a fuzzy love (feeling the affections but not living it out by doing loving deeds) is always a delicate one, but an absolutely essential one for the believer to maintain.