One of the great weaknesses of today's church is a tendency to denigrate and neglect the Old Testament. It's a much more sizable piece of literature than the New Testament, and it covers an enormous period of history, the history of redemption from the creation of the world until the appearance of the Messiah. All of that is a revelation of God's activity on this planet, and I believe it was inspired by the Holy Spirit and given to the church for the church's instruction and for the church's edification.
I also think that one of the great problems in today's church is an abysmal ignorance of God the Father. We relate to Jesus. He's our Redeemer. He's God in the flesh, so we have a way in which we can understand Jesus. It is more difficult when we look at God the Father and also the Holy Spirit. The history of the Old Testament certainly calls forth something of the Messiah who is to come, but it is constantly revealing the character of God the Father, the one who sends Jesus into this world, the one whom Jesus calls Father, the one from whom Jesus says he has been sent, that person to whom we are being reconciled and redeemed. So how can we possibly justify neglecting such an enormous body of literature that communicates to us the character, nature, and will of our Creator and the one who has sent our Redeemer to this planet?
Saint Augustine is the one who said that the New Testament is concealed in the Old Testament and the Old Testament is revealed by the New Testament. In fact, about three-fourths of the material of the New Testament is either a quotation from or allusion to what went before it. I don't think we can really understand the New Testament until we have made a very serious study of the Old Testament.
Obviously there are things in the Old Testament that do not apply to the Christian in our day. For example, we are not to continue the ceremonies that were required of the Jewish people; those ceremonies were "types" that anticipated the once-for-all fulfillment of them in the work of Christ. So for us to offer animals as sacrifices would be an insult to the completion of Jesus' work on the cross. That doesn't mean that since that part of the Old Testament is fulfilled we are to neglect it altogether. The Old Testament is a treasure-house of knowledge for the Christian who will seek to investigate it.