Staying on the Straight and Narrow
Some practical observations on best practices while preaching Christ from the OT are in order. Johnson helps preachers move from the OT through its own truths, through Christ’s fulfillment of that truth, and on to application to contemporary hearers. When interpreting an OT passage, preachers should 1) look for how that OT passage assigns spiritual significance to its people, event or institution; 2) discern how the passage may be portraying future redemptive events in “imagery drawn from God’s past deeds of creation and salvation”; and find out how the passage “testifies to the incompleteness of the redemption accessible through its own institutions, directing the longing of God’s people to a qualitatively superior future salvation/savior.”  These three features of OT Scripture we will call the OT truths in the schema below.
Johnson teaches preachers to do redemptive-historical christocentric preaching (and thus avoiding moralism and allegorism) by grasping an OT passage’s “symbolic depth in its own place in redemptive history… and consider how the event’s… original symbolic depth… finds final and complete fulfillment in Christ… [and then] identify and articulate how its message applies to ourselves and our twenty-first century listeners.” The schema Johnson presents can be illustrated as follows.
A) Consider the event or institution in the passage >
B) Consider the OT truth behind the event (symbolism) >
C) Consider the fulfillment of that OT truth in Christ (in the history of redemption and revelation) >
D) Make application to hearers in your preaching (significance) 
This schema is also useful in helping preachers avoid mere moralism and allegorism. For instance, allegory “draws direct symbolic connections between the historical details of an Old Testament event and our hearers’ experience of theology,” thus running directly from A to D. “The main difference between typological and allegorical interpretation,” says Greidanus, “is the way redemptive history functions in interpretation… redemptive history plays no role in interpreting Scripture [in allegory]. Typological interpretation, by contrast, requires redemptive history because the analogy and escalation between type and antitype are drawn within redemptive history.” (g., 91). Clowney helpfully illustrates a potential misuse of a text by allegory:
The typical aspects of Samson’s life are not to be found in the similarity of details. The gates of Gaza, removed by Samson to the heights of Hebron, cannot be identified directly with the gates of death. They are not in themselves symbolic. The structure that grounds the typology of the Old Testament narratives is the continuity of God’s work of redemption as it unfolds through history. The role of judge as a divinely endued and appointed deliverer anticipates the Judge who is yet to come.
Moralism likewise is a jump from step A through step B, across to step D—that is—moralism issues “ethical demands without grounding them in the gospel.” Greidanus quotes Luther as warning us to “beware of turning Christ into a Moses, as if He has nothing more for us than precept and example… before you take Christ as your example… recognize and accept Him as God’s gift to you.”
Thus Johnson offers helpful guidelines for preachers to think through an OT text, walking the tightrope of the OT event/person, through to its own truths/symbols, and on to how those truths are fulfilled in Christ and applicable to today’s hearers—avoiding slipping off the tightrope into moralism, on the one side, and allegorism, on the other.
 Johnson, Him We Proclaim, 219-230.
 ibid., 236-37.
 Adapted from ibid., 231.
 ibid., 232.
 Greidanus, Preaching Christ, 91. Yet—warns Greidanus—there is the danger of “typologizing, that is, overextending the use of typology by searching for types in rather incidental details in the text” which “in turn, can slip into allegorizing” (ibid., 97).
 Edmund P. Clowney, The Unfolding Mystery: Discovering Christ in the Old Testament: With Study and Application Questions (Phillipsburg, NJ: P & R Pub., 1999), 147 (emphasis mine).
 Johnson, Him We Proclaim, 233.
 Luther, Church Postil, as quoted in Greidanus, Preaching Christ, 119.