Objection 1. It would seem that the first man saw God through His Essence. For man's happiness consists in the vision of the Divine Essence. But the first man, "while established in paradise, led a life of happiness in the enjoyment of all things," as Damascene says (De Fide Orth. ii, 11). And Augustine says (De Civ. Dei xiv, 10): "If man was gifted with the same tastes as now, how happy must he have been in paradise, that place of ineffable happiness!" Therefore the first man in paradise saw God through His Essence.
Objection 2. Further, Augustine says (De Civ. Dei xiv, loc. cit.) that "the first man lacked nothing which his good-will might obtain." But our good-will can obtain nothing better than the vision of the Divine Essence. Therefore man saw God through His Essence.
Objection 3. Further, the vision of God is His Essence is whereby God is seen without a medium or enigma. But man in the state of innocence "saw God immediately," as the Master of the Sentences asserts (Sent. iv, D, i). He also saw without an enigma, for an enigma implies obscurity, as Augustine says (De Trin. xv, 9). Now, obscurity resulted from sin. Therefore man in the primitive state saw God through His Essence.
On the contrary, The Apostle says (1 Corinthians 15:46): "That was not first which is spiritual, but that which is natural." But to see God through His Essence is most spiritual. Therefore the first man in the primitive state of his natural life did not see God through His Essence.
I answer that, The first man did not see God through His Essence if we consider the ordinary state of that life; unless, perhaps, it be said that he saw God in a vision, when "God cast a deep sleep upon Adam" (Genesis 2:21). The reason is because, since in the Divine Essence is beatitude itself, the intellect of a man who sees the Divine Essence has the same relation to God as a man has to beatitude. Now it is clear that man cannot willingly be turned away from beatitude, since naturally and necessarily he desires it, and shuns unhappiness. Wherefore no one who sees the Essence of God can willingly turn away from God, which means to sin. Hence all who see God through His Essence are so firmly established in the love of God, that for eternity they can never sin. Therefore, as Adam did sin, it is clear that he did not see God through His Essence.
Nevertheless he knew God with a more perfect knowledge than we do now. Thus in a sense his knowledge was midway between our knowledge in the present state, and the knowledge we shall have in heaven, when we see God through His Essence. To make this clear, we must consider that the vision of God through His Essence is contradistinguished from the vision of God through His creatures. Now the higher the creature is, and the more like it is to God, the more clearly is God seen in it; for instance, a man is seen more clearly through a mirror in which his image is the more clearly expressed. Thus God is seen in a much more perfect manner through His intelligible effects than through those which are only sensible or corporeal. But in his present state man is impeded as regards the full and clear consideration of intelligible creatures, because he is distracted by and occupied with sensible things. Now, it is written (Ecclesiastes 7:30): "God made man right." And man was made right by God in this sense, that in him the lower powers were subjected to the higher, and the higher nature was made so as not to be impeded by the lower. Wherefore the first man was not impeded by exterior things from a clear and steady contemplation of the intelligible effects which he perceived by the radiation of the first truth, whether by a natural or by a gratuitous knowledge. Hence Augustine says (Gen. ad lit. xi, 33) that, "perhaps God used to speak to the first man as He speaks to the angels; by shedding on his mind a ray of the unchangeable truth, yet without bestowing on him the experience of which the angels are capable in the participation of the Divine Essence." Therefore, through these intelligible effects of God, man knew God then more clearly than we know Him now.
Reply to Objection 1. Man was happy in paradise, but not with that perfect happiness to which he was destined, which consists in the vision of the Divine Essence. He was, however, endowed with "a life of happiness in a certain measure," as Augustine says (Gen. ad lit. xi, 18), so far as he was gifted with natural integrity and perfection.
Reply to Objection 2. A good will is a well-ordered will; but the will of the first man would have been ill-ordered had he wished to have, while in the state of merit, what had been promised to him as a reward.
Reply to Objection 3. A medium (of knowledge) is twofold; one through which, and, at the same time, in which, something is seen, as, for example, a man is seen through a mirror, and is seen with the mirror: another kind of medium is that whereby we attain to the knowledge of something unknown; such as the medium in a demonstration. God was seen without this second kind of medium, but not without the first kind. For there was no need for the first man to attain to the knowledge of God by demonstration drawn from an effect, such as we need; since he knew God simultaneously in His effects, especially in the intelligible effects, according to His capacity. Again, we must remark that the obscurity which is implied in the word enigma may be of two kinds: first, so far as every creature is something obscure when compared with the immensity of the Divine light; and thus Adam saw God in an enigma, because he saw Him in a created effect: secondly, we may take obscurity as an effect of sin, so far as man is impeded in the consideration of intelligible things by being preoccupied with sensible things; in which sense Adam did not see God in an enigma.
Source: Summa Theologica (New Advent)