In Book II, the first chapter almost seems to be a continuation of Book I, where the rule of interpretation presented in that Book is discussed once more. The application of this can basically be called the “form-of-a-servant” rule and the “form-of-God” rule. We speak of Christ in either of these manners, for He has two natures by His incarnation.
It appears, however, that this continuation makes sense, for the rule is here applied to the Son and the Spirit “as sent.” The Father, however, is never said to be sent, but is rather the one who sends (later Augustine will certainly address that the Father and Son both send the Spirit). This is the required preface to the rest of Book II, which begins to look at the Old Testament theophanies.
Here, we look at the questions of whether it was one Person or another who appeared in the Old Testament when God spoke to men, or if it was whether the Trinity as a whole (this “Trinity as a whole” phrase certainly does not mean there can be any division into “parts;” remember we are speaking on the edge of human language). After investigating many instances of God’s meeting with man in the Hebrew Scriptures, including those of the garden of Eden, the appearance of the three to Abraham, and the various appearances to Moses, Augustine concludes the following:
“Wherefore, since in that our threefold division we determined to inquire, first, whether the Father, or the Son, or the Holy Spirit; or whether sometimes the Father, sometimes the Son, sometimes the Holy Spirit; or whether, without any distinction of persons, as it is said, the one and only God, that is, the Trinity itself, appeared to the fathers through those forms of the creature: now that we have examined, so far as appeared to be sufficient what places of the Holy Scriptures we could, a modest and cautious consideration of divine mysteries leads, as far as I can judge, to no other conclusion, unless that we may not rashly affirm which person of the Trinity appeared to this or that of the fathers or the prophets in some body or likeness of body, unless when the context attaches to the narrative some probable intimations on the subject. For the nature itself, or substance, or essence, or by whatever other name that very thing, which is God, whatever it be, is to be called, cannot be seen corporeally: but we must believe that by means of the creature made subject to Him, not only the Son, or the Holy Spirit, but also the Father, may have given intimations of Himself to mortal senses by a corporeal form or likeness.”