Typology highlights the patterns that God uses throughout Salvation History to provide context for our understanding of the identity and mission of Jesus. Typology understands that the Old Testament serves as a set of blueprints for understanding the Gospel of Christ. Blueprints represent the real but are not the real. They are a model, a representation, which helps us to understand the real. Before a building exists the blueprints are the closest things we have to the real building. Once the building is built, the blueprints are no longer the closest representation of the reality they describe. The building is. So it is with Christ. Typology uses the patterns of the Old Testament as a guide to understanding the reality of Jesus.
Whereas allegories are dangerous because they leverage superficial similarities, types are beneficial because they seek out the deep theological connection. An allegorical reading of the Bible will impregnate the text with meaning that is arbitrary and needlessly creative. For example, the five smooth stones that David picks up to slay Goliath in 1 Samuel 17:40 is often preached as an allegory. That is, the preacher might say that the five stones represent the five books of the Torah, or the four Gospels and Acts, or love, joy, peace, patience, and kindness. All of these allegorical readings are dangerous and not typological. Why does the text tell us that David picked up five smooth stones? Probably because that is what David did. It is unfortunate that many preachers are afraid to preach typologically because of allegorical abuses through the centuries.
To preach typologically, the preacher must identify correspondence, escalation, and theocentricity in both the type and anti-type. Correspondence means there must be common elements between the type and anti-type. Escalation means that the corresponding elements must be greater, i.e. more acute, in the anti-type than in the type. And theocentricity means that both type and anti-type must represent a similar aspect of God’s redemptive activity. The Passover lamb is a good example of typology. As a type, the Passover lamb corresponds to Jesus, the anti-type, because both are slaughtered as a sacrifice of deliverance. The crucifixion of Jesus is an escalation of the slaughter of the Passover lamb by the very fact that Jesus is not an animal but is both human and divine. Moreover, the crucifixion of Jesus is once-for-all and is sufficient for all people who believe. Neither of those statements is true of the type. This typology is theocentric because both the lamb and Jesus are killed so that the wrath of God might pass over those who apply the blood appropriately.
Typology informs our understanding of both the identity and mission of Jesus by providing us with shadows, pictures, and object lessons for Christ in the Old Testament.
 Clowney (Preaching Christ in All of Scripture, 31) provides a strong and convincing counter to those who would argue that only types identified in the New Testament should be identified and preached: “Warned by the arbitrary allegorizing of Origen, Reformed expositors have often shied away from typology. My own seminary teacher instructed us to recognize as types in the Old Testament only those things that are identified as types in the New Testament. That is certainly a safe rule. If the New Testament specifies something as a type, we may so interpret it. But that is a little like saying that you can find solutions to math problems only by looking in the back of the book, since you haven’t a clue as to how to work the problems. To conclude that we can never see a type where the New Testament does not identify it is to confess hermeneutical bankruptcy. We know that New Testament writers did find types, but we confess that we cannot learn how they did it.”