1. I Would have them believe, who are willing to do so, that I had rather bestow labor in reading, than in dictating what others may read. But let those who will not believe this, but are both able and willing to make the trial, grant me whatever answers may be gathered from reading, either to my own inquiries, or to those interrogations of others, which for the character I bear in the service of Christ, and for the zeal with which I burn that our faith may be fortified against the error of carnal and natural men, I must needs bear with; and then let them see how easily I would refrain from this labor, and with how much even of joy I would give my pen a holiday. But if what we have read upon these subjects is either not sufficiently set forth, or is not to be found at all, or at any rate cannot easily be found by us, in the Latin tongue, while we are not so familiar with the Greek tongue as to be found in any way competent to read and understand therein the books that treat of such topics, in which class of writings, to judge by the little which has been translated for us, I do not doubt that everything is contained that we can profitably seek; while yet I cannot resist my brethren when they exact of me, by that law by which I am made their servant, that I should minister above all to their praiseworthy studies in Christ by my tongue and by my pen, of which two yoked together in me, Love is the charioteer; and while I myself confess that I have by writing learned many things which I did not know: if this be so, then this my labor ought not to seem superfluous to any idle, or to any very learned reader; while it is needful in no small part, to many who are busy, and to many who are unlearned,and among these last to myself. Supported, then, very greatly, and aided by the writings we have already read of others on this subject, I have undertaken to inquire into and to discuss, whatever it seems to my judgment can be reverently inquired into and discussed, concerning the Trinity, the one supreme and supremely good God; He himself exhorting me to the inquiry, and helping me in the discussion of it; in order that, if there are no other writings of the kind, there may be something for those to have and read who are willing and capable; but if any exist already, then it may be so much the easier to find some such writings, the more there are of the kind in existence.
2. Assuredly, as in all my writings I desire not only a pious reader, but also a free corrector, so I especially desire this in the present inquiry, which is so important that I would there were as many inquirers as there are objectors. But as I do not wish my reader to be bound down to me, so I do not wish my corrector to be bound down to himself. Let not the former love me more than the catholic faith, let not the latter love himself more than the catholic verity. As I say to the former, Do not be willing to yield to my writings as to the canonical Scriptures; but in these, when thou hast discovered even what thou didst not previously believe, believe it unhesitatingly; while in those, unless thou hast understood with certainty what thou didst not before hold as certain, be unwilling to hold it fast: so I say to the latter, Do not be willing to amend my writings by thine own opinion or disputation, but from the divine text, or by unanswerable reason. If thou apprehendest anything of truth in them, its being there does not make it mine, but by understanding and loving it, let it be both thine and mine; but if thou convictest anything of falsehood, though it have once been mine, in that I was guilty of the error, yet now by avoiding it let it be neither thine nor mine.
3. Let this third book, then, take its beginning at the point to which the second had reached. For after we had arrived at this, that we desired to show that the Son was not therefore less than the Father, because the Father sent and the Son was sent; nor the Holy Spirit therefore less than both, because we read in the Gospel that He was sent both by the one and by the other; we undertook then to inquire, since the Son was sent thither, where He already was, for He came into the world, and |was in the world;| since also the Holy Spirit was sent thither, where He already was, for |the Spirit of the Lord filleth the world, and that which containeth all things hath knowledge of the voice;| whether the Lord was therefore |sent| because He was born in the flesh so as to be no longer hidden, and, as it were, came forth from the bosom of the Father, and appeared to the eyes of men in the form of a servant; and the Holy Spirit also was therefore |sent,| because He too was seen as a dove in a corporeal form, and in cloven tongues, like as of fire; so that, to be sent, when spoken of them, means to go forth to the sight of mortals in some corporeal form from a spiritual hiding-place; which, because the Father did not, He is said only to have sent, not also to be sent. Our next inquiry was, Why the Father also is not sometimes said to be sent, if He Himself was manifested through those corporeal forms which appeared to the eyes of the ancients. But if the Son was manifested at these times, why should He be said to be |sent| so long after, when the fullness of time was come that He should be born of a woman; since, indeed, He was sent before also, viz., when He appeared corporeally in those forms? Or if He were not rightly said to be |sent,| except when the Word was made flesh; why should the Holy Spirit be read of as |sent,| of whom such an incarnation never took place? But if neither the Father, nor the Son, but the Holy Spirit was manifested through these ancient appearances; why should He too be said to be |sent| now, when He was also sent before in these various manners? Next we subdivided the subject, that it might be handled most carefully, and we made the question threefold, of which one part was explained in the second book, and two remain, which I shall next proceed to discuss. For we have already inquired and determined, that not only the Father, nor only the Son, nor only the Holy Spirit appeared in those ancient corporeal forms and visions, but either indifferently the Lord God, who is understood to be the Trinity itself, or some one person of the Trinity, whichever the text of the narrative might signify, through intimations supplied by the context.