Preach the Christian Life from the Old Testament

If we are going to be faithful to God's Word, we are going to preach the Christian Life to Christians from the Old Testament (as well as the New); we will understand and fully appreciate the glorious new covenant aspects of the doctrine of sanctification; we will not try to anachronistically read them back into the old covenant text. But when applying the old covenant text to new covenant believers, we will not fail to bring its moral exhortation to bear on their hearts and lives, in light of the glorious new covenant realities of which they blessedly partake.
J. Ligon Duncan, Preaching The Cross (pg 64)