Teaching and love
For Augustine, the end of Christian doctrine, as with everything else, is love.
Why does God teach his people through the fallible words of earthly men, rather than teaching each individual everything he needs directly? According to Augustine, it is so that he can build us up as a temple:
And all of these things in both instances [the conversions of Paul and Cornelius] might have been done by an angel, but the condition of man would be lowered if God had not wished to have men supply His word to men. How would there be truth in what is said—“For the temple of God is holy, which you are”—if God did not give responses from a human temple, but called out all that He wished to be taught to men from Heaven and through angels? For charity itself, which holds men together in a knot of unity, would not have a means of infusing souls and almost mixing them together if men could teach nothing to men. [Augustine, On Christian Doctrine, Preface. Translated by D.W. Robinson, Jr.]
The Church is not merely a means of transmitting information or even salvific benefits to individual people. If God wanted only to do that, he could have done it himself. Rather, the whole point is to build up a human social body over time.
The body of a single person is not merely a mechanism for providing benefits to each of the hands, feet and other members; rather, the body is the common entity towards which each member offers its unique virtue, and from which it is nourished with what it needs to make those offerings. The body is more than the sum of its parts. Likewise, the end of God’s work in building the church is to build the church, that is, to prepare a bride, a body for his human Son. The church is the body of Christ, “which has many members performing diverse offices, in a bond of unity and charity which is, as it were, its health” (Bk. I, ch. xvi). To prepare that body, he needs to knit together the members in the unity of the Spirit in the bond of peace. One of the central ways he does that knitting-together is through teaching.
When someone teaches well, they give not merely the teaching, but themselves. Teaching done well can stir up in the students personal affection for the teacher and a kind of loyalty to him; likewise, a teacher can come to love his students, especially if he sees his teaching come to grow and shape them.
The apostles, prophets, evangelists and pastor-teachers were given as gifts to the church for its building-up. By implication, then, these various ministers were also given so that the church would come to love them, and likewise, that the ministers in their teaching would grow in their affection for the saints. So Paul loved and gave himself to the Galatians, and at least for a time, they would have torn out their own eyes for his sake.
Augustine’s line of reasoning implies that, without this bond of love forged by teaching, the church could not actually be the temple in which the Spirit of God dwells. God has arranged this building project in such a way that the stones of the temple are not merely passive materials being worked upon, but as living stones, they are active participants in the construction.