Jaltus and Arminian,
I think that Jesus' conversation with Nicodemus when he came to talk to Jesus during the night throws some light on just how close the OC and NC are.
Jesus rebuked Nicodemus for not understanding that a man must be born again to see the kingdom of heaven. Jesus actually said, How can you be a Master in Isreal and not know this? If the new birth, and thus the NC, should have been known to Nicodemus this makes both Covenants very similar, if not identical for Nicodemus was a man highly educated in the Jewish knowledge and theology. Why would Jesus have rebuked Nicodemus for not knowing something that wasn't taught in the OT? Wouldn't that be completely out of character for Him? Rather hypocritical?
While the term "new birth" is not seen in the OT, the idea is clearly shown in David's prayer for a clean heart and right spirit, and that God not take His Holy Spirit away from him (David). The multiple promises in the OT concerning a new heart and being able to walk in the commandments of God, and do them most surely point to the covenants being pretty much identical. Paul's statements about faith also point in this same direction. In fact, when the entire NT is studied from this perspective the Bible becomes a complete whole, not a book artificially divided between Malachi and Matthew.
Also, is not what Jesus says to the sheep and goats in Matthew 25 not based upon the same principles found in Isaiah 58? There God points out what He really wanted from those of Isaiah's time, instead of their legalistic dependence upon the perversion of the ceremonies that God gave them in the sin offerings and Sactuary Services, while they did the same works as the Pharisees of Jesus' time. Look at Isaiah 58 closely and then look at how the Pharisees behaved. You will find great similarities, if not a repetition of the same exact sins. Funny how God condemned the legalism in the OT and the NT while pointing out that what He really wanted was a heart religion in both places. Why would He have done this if the two Covenants didn't have the same basis?
The Sanctuary services pointed forward to Jesus. Thus the offering of a sacrifice for sin showed faith in the Redeemer to come. Faith in Christ and His sacrifice has always been the condition for salvation.