The Law of Moses was necessary!

The Old Testament saints also walked with God and had fellowship with Him. The Law of Moses was given not to bring them to salvation. They had to believe in God and receive forgiveness. However, they also had to walk in the Law of Moses and do as commanded because God had to reveal Himself as Holy before He could reveal His grace. How could God reveal Himself as merciful if they did not know that their actions brought death? Sin had to revealed as sin before it could be forgiven. Performing the works of the Law did not lead them to God but had to be done also. David did so in 1 Chr 21:26. The Law was supposed to be a way of life for them even though it was also ceremonial at times. That is why Peter calls it a burden that was hard to bear and asked the others why they wanted the Gentiles to also follow this routines. Acts 15:10.
It is interesting to note however that David after he committed the sin of adultery and willful murder could not take shelter in the law of Moses which would have condemned him to death. He cried out for forgiveness and was granted a reprieve. What I want to point out is that God was revealing the same kind of grace He revealed in the New. He is the same yesterday, today and forever.