Chapter V
16 As to the spiritual body which we shall have in the resurrection, how great a change for the better it is to undergo -- whether it shall become pure spirit, so that the whole man shall then be a spirit, or shall (as I rather think, but do not yet confidently maintain) become a spiritual body in such a way as to be called spiritual because of a certain ineffable facility in its movements, but at the same time to retain its material substance, which cannot live and feel by itself, but only through the spirit which uses it (for in our present state, in like manner, although the body is spoken of as animated [animal], the nature of the animating principle is different from that of the body),and whether, if the properties of the body then immortal and incorruptible shall remain unchanged, it shall then in some degree aid the spirit to see visible, i,e. material things, as at present we are unable to see anything of that kind except through the eyes of the body, or our spirit shall then be able, even in its higher state, to know material things without the instrumentality of the body (for God Himself does not know these things through bodily senses),on these and on many other things which may perplex us in the discussion of this subject, I confess that I have not yet read anywhere anything which I would esteem sufficiently established to deserve to be either learned or taught by men.
17. And for this reason, if our brother will J bear patiently any degree whatever of hesitation I on my part, let us in the meantime, because of] that which is written, "We shall see Him as, He is," prepare, so far as with the help of God, Himself we are enabled, hearts purified for that vision. Let us at the same time inquire more calmly and carefully concerning the spiritual body, for it may be that God, if He know this to be useful to us, may condescend to show us some definite and clear view on the subject, in accordance with His written word. For if a more careful investigation shall result in the discovery that the change on the body shall be so great that it shall be able to see things that are invisible, such power imparted to the body will not, I think, deprive the mind of the power of seeing, and thus give the outward man a vision of God which is denied to the inward man; as if, in contradiction of the plain words of' Scripture, "that God may be all and in all," God were only beside the man -- without him, and not in the man, in his inner being; or as if He, who is everywhere present in his entirety, unlimited in space, is so within man that He can be seen outside only by the outward man, but cannot be seen inside by the inward man. If such opinions are palpably absurd -- for, on the contrary, the saints shall be full of God; they shall not, remaining empty within, be surrounded outside by Him; nor shall they, through being blind within, fail to see Him of whom they are full, and, having eyes only for that which is outside of themselves, behold Him by whom they shall be surrounded -- if, I say, these things are absurd, it remains for us to rest meanwhile certainly assured as to the vision of God by the inward man. But if, by some wondrous change, the. body shall be endowed with this power, another new faculty shall be added; the faculty formerly possessed shall not be taken away.
18. It is better, then, that we affirm that concerning which we have no doubt -- that God shall be seen by the inward man, which alone is able, in our present state, to see that love in commendation of which the apostle says, "God is love;" the inward man, which alone is able to see "peace and holiness, without which no man shall see the Lord." For no fleshly eye now sees love, peace, and holiness, and such things; yet all of them are seen, so far as they can be seen, by the eye of the mind, and the purer it is the more clearly it sees; so that we may, without hesitation, believe that we shall see God, whether we succeed or fail in our investigations as to the nature of our future body -- although, at the same time, we hold it to be certain that the body shall rise again, immortal and incorruptible, because on this we have the plainest and strongest testimony of Holy Scripture: If, however, our brother affirm now that he has arrived at certain knowledge as to that spiritual body, in regard to which I am only inquiring, he will have just cause to be displeased with me if I shall refuse to listen calmly to his instructions, provided only that he also listen calmly to my questions. Now, however, I entreat you, for Christ's sake, to obtain his forgiveness for me for that harshness in my letter, by which, as I have learned, he was, not without cause, offended; and may you, by God's help, cheer my spirit by your answer.
Source: Letters (New Advent)