One of THE single most important theological and pastoral issues is the relationship between Christian living and the law.
Here's ten words giving some of my reasons why I reject an absolute antithesis between law and gospel and some sort of summary of how I think we should relate to the OT law.
1. The law was never intended as a system of meriting salvation. In fact, the sacrificial system and priesthood at the centre of the law were about grace, forgiveness, propitiation.
2. One way of summarising the gospel is 'Jesus Christ is Lord.' This means immediately that the concept of 'law' (i.e. commands you really must obey) is included within the gospel itself. I can't say I believe that Jesus is Lord and then totally willfully persistently ignore everything the Lord wants me to do. It just doesn't make sense.
3. Then, add to 2. the fact that obedience to Jesus Christ is often described using commands from the Old Testament law (yes, modified and transformed by the salvation-history context)...
4. ...then add to 2. and 3. the fact that the Christian's love-response to Jesus Christ is described as fulfilling the law...
5. ....then add to 2., 3. and 4. the fact that Christ himself thought he was fulfilling the law in his life and in his teaching....
6. ...and what do you get? I think you get the idea that the Christian is, although not under the law (i.e. we don't live under the Mosaic administration of the covenant, we are not living between Sinai and the cross, but between the cross and eternity), under Christ, but, that being under Christ means being under someone whose commands are the fulfillment of the law.
7. In other words Christians should relate to the Mosaic law in the same basic way that we relate to all of the Old Testament - through Christ. The law comes to us, not in exactly the same way it did to the Israelites as they stood at the foot of Mount Sinai, but transformed, renewed, changed, by the fact that the law has found its climax in the life and work and revelation of Jesus Christ the Lord. The law is for us because it speaks of Christ, by showing us our need of him by exposing sin, by helping us fill out the content of what obedience to Christ looks like, etc. The law is there to instruct and train us in righteous living.
8. Point 7. will look quite different depending (in part) on the law in question. The way I approach the food laws of Leviticus will look quite different to the way I approach 'do not murder' but in neither case can I write off the law as irrelevant to Christian living, nor can I simply lift the law straight out of the Old Testament without reflection on the fact that Christ has brought things to a climax.
9. 'Spirit vs. law' is ultimately a non-starter too, since the Spirit writes the law on our hearts.
10. Were the writers of psalm 19 and psalm 119 simply off their heads, or were they the worst sort of pharisaical legalists? Or could it be that for the living-by-grace Christian the law is a great friend since it communicates the wise requirements of our loving and holy King?