Immanuel Kant wrote, ‘Reason, with its practical law, determines the will immediately, not by means of an intervening feeling of pleasure or pain, not even of pleasure in the law itself, and it is only because it can, as pure reason, be practical, that it is possible to be legislative.’ (Critique of Practical Reason)
He is saying that reason must ignore feelings, in order to come to true universal moral conclusions.
But on the contrary, Adam and Eve’s feelings and inclinations were originally designed (pre-Fall), to point them always away from sin, and towards pleasing God. They needed to follow these inclinations and desires. Indeed, their inclinations were their only source of knowledge of moral truth (other than the explicit commands about the tree of knowledge). Adam and Eve needed no detailed rule book to live well.
Nor did they need to follow ‘practical reason’ to arrive at basic moral truth. For since the law which must be known to please God is so detailed (the Bible gives many moral laws), and since there is no viable way to arrive at all these laws by reason alone, it follows that this vast moral law must have been accessible to Adam and Eve apart from reason.
This mechanism to arrive at this moral law is the same in all people today (although marred by sin), for we share Adam and Eve’s humanity.
This is not to say that reason has no role in morality at all. For example, reason has ever been required to come to second level moral principles, e.g.
If I am a child, and am told by my Father to watch my brother, I should stay within view of my brother, since I know that I should obey my Father. The requirement to stay within view of my brother is arrived at by reason, but the the moral requirement to obey my Father is not.
As for ‘primary’, ‘basic’ moral truth, the mechanism all humans have for knowing it is not practical reason, but the moral inclinations which come to us, as part of our design.
That our inclinations now bend away from truth, because of our sin, does not change the fact that our inclinations are the only universally available source of morality.
There is no universally legislative practical reason, as Kant describes it.