When the Lord said to him [Adam], "In the day that thou eatest thereof, thou shalt surely die" (Genesis 2:17), it implied, "If thou eat not of it, thou shalt surely live." Besides, the tree of life, which was one of the seals of that covenant, serves to evince the same thing. It sealed the promise of life to Adam as long as he continued to perform perfect obedience.
It is evident that the infinitely great and sovereign Creator could be under no obligations to man, the creature of his power, but such as arose from the wisdom, goodness, and faithfulness of His own nature. It was therefore free to Him whether He would still, by absolute authority, command man to obey Him, or enter into a covenant with man for that purpose; whether after perfect obedience to His law He would give man eternal life or annihilate him; and whether, if it should please Him to give it, He would bestow it on condition of man's obedience, or make a free grant of it to him, and confirm him in the eternal enjoyment of it, as He has done elect angels. It depended solely upon the will of God whether there would be a covenant at all containing a promise of eternal life to man, and, if a promise of it, whether that promise could be absolute or conditional. The promise of eternal life upon man's perfect obedience, the, flowed entirely from the good pleasure and free grace of God. Had Adam fulfilled the condition of life in the first covenant, the Lord, instead of having been a debtor to him for his obedience, would have been a debtor only to His own grace and faithfulness in the promise. It is manifest, the, that there could have been no real merit in the perfect obedience of man, nor so much as the smallest proportion between it and the promised reward. If Adam had performed the condition of that covenant, he could not have expected eternal life upon any ground except this: that God had graciously promised it on that condition.A Treatise on The Law and The Gospel by John Colquhoun, pp 16-17