In Him We Proclaim: Preaching Christ from All the Scriptures, Dennis E. Johnson points out the weaknesses in moralistic/exemplaristic of OT saints, being a direct model for New Covenant believers absent of the person and work of Jesus Christ.
The great weakness of moralistic, exemplaristic preaching is its tendency to enlist Old Testament examples in order to lay ethical obligations on hearers without showing how Christ kept covenant faithfulness where the negative examples failed, and how Christ’s perfect righteousness fulfills even the best obedience offered by the Old Testament’s most positive examples. By “cutting the corner” and bypassing the text’s “fulfillment in Christ” …in a desire to show the text’s relevance to hearers’ daily struggles and relationships, moralism excises from the biblical narratives the source of their life-changing power, which is their testimony to the saving mercy of God in the obedience and sacrifice of Jesus. When Christ and his all-fulfilling role as Lord and Servant of the covenant are left out of the “equation” the narratives of Noah, Jacob, Joseph, Samson, David, and Nehemiah are subtly transformed from gospel into law: “Do this and live”; “Imitate X and live.” The law, however, as “holy and righteous and good” as it is as a divine standard (Rom. 7:12) is weakened by our fallen, sinful nature and cannot impart the life and spiritual strength needed for us to obey its commands or emulate its positive exemplars (Rom. 8:3; Gal. 3:2 1; 2 Cor. 3:6-9). (p. 234)
Since the grace of the exodus set the context for the stipulations that Israel was to observe as the Lord’s servant, how much more should Christian preachers expound those many biblical texts that shine the spotlight on the responsibilities of God’s covenant servants (whether commandments, wisdom maxims, or narratives that profile faithful or unfaithful responses to the Lord of the covenant) by calling attention to God’s gracious provision of Jesus, the Servant who kept covenant commandments and bore covenant curse in our place! But our exposition of imperative texts does not stop with what Christ has done for us; it also extends to what Christ, by his Word and Spirit, is doing in us. In the context of his achievement of our redemption, the Spirit’s gracious, persistent application of redemption in our sanctification is good news as well. (p. 367)