Genesis 2:16-17

For in the beginning of the world He gave to Adam himself and Eve a law, that they were not to eat of the fruit of the tree planted in the midst of paradise; but that, if they did contrariwise, by death they were to die. Which law had continued enough for them, had it been kept. For in this law given to Adam we recognise in embryo all the precepts which afterwards sprouted forth when given through Moses; that is, You shall love the Lord your God from your whole heart and out of your whole soul; You shall love your neighbour as yourself; You shall not kill; You shall not commit adultery; You shall not steal; False witness you shall not utter; Honour your father and mother; and, That which is another's, shall you not covet. For the primordial law was given to Adam and Eve in paradise, as the womb of all the precepts of God. In short, if they had loved the Lord their God, they would not have contravened His precept; if they had habitually loved their neighbour—that is, themselves—they would not have believed the persuasion of the serpent, and thus would not have committed murder upon themselves, by falling from immortality, by contravening God's precept; from theft also they would have abstained, if they had not stealthily tasted of the fruit of the tree, nor had been anxious to skulk beneath a tree to escape the view of the Lord their God; nor would they have been made partners with the falsehood-asseverating devil, by believing him that they would be "like God;" and thus they would not have offended God either, as their Father, who had fashioned them from clay of the earth, as out of the womb of a mother; if they had not coveted another's, they would not have tasted of the unlawful fruit.
Therefore, in this general and primordial law of God, the observance of which, in the case of the tree's fruit, He had sanctioned, we recognise enclosed all the precepts specially of the posterior Law, which germinated when disclosed at their proper times. For the subsequent superinduction of a law is the work of the same Being who had before premised a precept; since it is His province withal subsequently to train, who had before resolved to form, righteous creatures. [Answer to the Jews 2]

For it was a most benignant act of His thus to point out the issues of transgression, lest ignorance of the danger should encourage a neglect of obedience. Now, since it was given as a reason previous to the imposition of the law, it also amounted to a motive for subsequently observing it, that a penalty was annexed to its transgression; a penalty, indeed, which He who proposed it was still unwilling that it should be incurred. Learn then the goodness of our God amidst these things and up to this point; learn it from His excellent works, from His kindly blessings, from His indulgent bounties, from His gracious providences, from His laws and warnings, so good and merciful. [Against Marcion 2.4]

Adam had received from God the law of not tasting "of the tree of recognition of good and evil," with the doom of death to ensue upon tasting. However, even (Adam) himself at that time, reverting to the condition of a Psychic after the spiritual ecstasy in which he had prophetically interpreted that "great sacrament" with reference to Christ and the Church, and no longer being "capable of the things which were the Spirit's," yielded more readily to his belly than to God, heeded the meat rather than the mandate, and sold salvation for his gullet! He ate, in short, and perished; saved (as he would) else (have been), if he had preferred to fast from one little tree: so that, even from this early date, animal faith may recognise its own seed, deducing from thence onward its appetite for carnalities and rejection of spiritualities. I hold, therefore, that from the very beginning the murderous gullet was to be punished with the torments and penalties of hunger. Even if God had enjoined no preceptive fasts, still, by pointing out the source whence Adam was slain, He who had demonstrated the offence had left to my intelligence the remedies for the offence. [On Fasting 3]

Knowing once more how the will of man could sway to either side, in anticipation God secured the grace given them by a command and by the place where he put them. For he brought them into his own garden and gave them a law so that, if they kept the grace and remained good, they might still keep the life in paradise without sorrow or pain or care, besides having the promise of incorruption in heaven. But if they transgressed and turned back and became evil, they might know that they were incurring that corruption in death that was theirs by nature, no longer to live in paradise but cast out of it from that time forth to die and abide in death and corruption.

In the very midst he planted
the Tree of Knowledge,
endowing it with wonder,
hedging it in with dread,
so that it might straightway serve
as a boundary to the inner region of paradise.
Two things did Adam hear
in that single decree:
that they should not eat of it
and that, by shrinking from it,
they should perceive that it was not lawful
to penetrate further, beyond that tree.

Having spoken of the introduction of Adam into Paradise and the reason God brought him there, Scripture turns to describe the commandment which was laid down for him, as follows: "And the Lord God commanded Adam, saying: 'You may indeed eat of all the trees in Paradise, but of the Tree of Knowledge of good and evil you shall not eat; for on the day you eat of it you will certainly die.' " [ Gen. 2:16-17 ]

This commandment was a light one, for God had given him the whole of Paradise and held back from him but a single tree. If one tree sufficed for someone's sustenance, and many trees were withheld from him, there would still be relief for his distress, seeing that there still existed food for his hunger. But where it is a case of God's giving him many trees when one would have been sufficient, this means that if transgression takes place, it is not as a result of any real need, but because of contempt. So God withheld from him a single tree, hedging it around with death, so that even if Adam were to fail to keep the law out of love for the Lawgiver, at least the fear of death that surrounded the tree would make him afraid of overstepping the law.