If the just demerit and wages of sin was contained in the threatening (as no doubt it was) it could be no less than an eternal punishment that was threatened. For if that is not the desert of every sin, it cannot be due to any sin. The reason why the punishment of any sin is eternal is so that the penalty inflicted on the sinner may be adequate to the offence. The punishment has an infinity in its eternity, because the fault is infinitely aggravated, and that can only be in regard of its object. There is nothing that can be an infinite aggravation of sin but its being committed against a God of infinite greatness, glory, and goodness. And this aggravation attends every sin, as it is sin against God, and though other circumstances may increase the provocation and so intend the degree of the sinner’s pain, yet none but this can reach infinity. The punishment therefore due to Adam for sin against God could be no other or no less than eternal death, which is that intended in the sanction of the law given to him.
Nehemiah Coxe, From Adam to Christ