Central to the hermeneutic of St. Augustine is what is known as the totus Christus hermeneutic, the hermeneutical principle that understands Sacred Scripture to be speaking of the whole Christ, head and body, Jesus and the Church. Thus, the Bible is both Christocentric and ecclesiocentric. He summarizes here:
… knowing as we do that the head and the body- that is, Christ and His Church- are sometimes indicated to us under one person (for it is not in vain that it is said to believers, You then are Abraham’s seed [Gal. 3:29], when there is but one seed of Abraham, and that is Christ), we need be in a difficulty when a transition is made from the head to the body or from the body to the head, and yet no change made in the person spoken of. For a single person is represented as saying, He has decked me as a bridegroom with ornaments, and adorned me as a bride with jewels and yet it is, of course, a matter for interpretation which of these two refers to the head and which to the body, that is, which to Christ and which to the Church.
-St. Augustine, On Christian Doctrine, 3.31.44 (as quoted in Peter J. Leithart, Deep Exegesis)
So, when you read of Yahweh’s chosen servant in Isaiah, you are definitely reading about Jesus of Nazareth. Yet Paul, preaching in Antioch of Pisidia, can quote Isaiah 49:6 where Yahweh tells his servant Jesus “I have made you a light for the Gentiles, that you may bring salvation to the ends of the earth” and say “…so the Lord has commanded us.” (Acts. 13:47) The Christ spoken of by Isaiah and Paul is the whole Christ, head and body. The Church is brought through baptism into union with Jesus, and thus becomes members of Christ. In his death, we die; in his resurrection, we are raised (Rom. 6.) We are called as disciples, then, to live cross-shaped lives of self-giving, and to bring the reality of the resurrection to bear on the world.
In Peter J. Leithart’s important work, Deep Exegesis: The Mystery of Reading Scripture, he suggests that the “breakdown of this Augustinian hermeneutical principle has been one of the main sources of interpretive confusion among Protestants.” (p.174.) Some only see Jesus in the text, and thus allow no direct application to the Christian life. Others only find moral lessons and rule out any possibility of christological typology. A recovery of the totus Christus, however, will lead us to find our practical tropology flowing out of the christological typology. Indeed, when we see the whole Christ in the text, we learn to read like St. Paul and like Jesus.