David Murray, professor of Old Testament and practical theology at Puritan Reformed Theological Seminary in Grand Rapids, Michigan, answers this question when asked about preaching Christ from the Old Testament:
I’d also like to encourage preachers and teachers to be clear and consistent on the question: “How were Old Testament believers saved?” The most common options seem to be:
1. They were saved by obeying the law.
2. They were saved by offering sacrifices.
3. They were saved by a general faith in God.
Unless we consistently answer #4, we end up portraying heaven as not only populated by lovers of Christ, but also by legalists, ritualists, and mere theists who never knew Christ until they got there. Turning back again in order to go forwards, may I recommend Calvin’s Institutes Book 2 (chapters 9-11) to help remove some of the blur that often surrounds this question.
Read the whole post to see thoughts from Murray, as well as Tim Keller and Don Carson, about some cautions when preaching Christ from the Old Testament.