Therein lies the mystery of the tree of "the knowledge of good and evil."
There are two views you can take about morality in relation to God:
1. Good is good because God proclaimed it so.
2. Good is good in and of itself, and God is following goodness.
Adam and Eve already had a conscience--you might say with one and only one law (don't eat of the fruit of that tree). This corresponds to #1.
Or you might say that Adam and Eve had no conscience; it was the breaking of the Covenant that created the conscience. This corresponds to #2.
The Calvinists, I think, thought that the story of the tree was meant to illustrate that it is both.