Death is not natural but arises from the violation of the divine commandment (Gen. 2:17); from the devil insofar as he by his seduction caused humanity to fall and die (John 8:44); from sin itself inasmuch as it has a disintegrating impact on the whole of human life and, as it were, produces death from within itself (James 1:15); and from the judgment of God since he pays the wages of sin in the currency of death (Rom. 6:23). And in Scripture this death is never identical with annihilation, with nonbeing, but always consists in the destruction of harmony, in being cut off from the various life settings in which a creature has been placed in keeping with one’s nature, in returning to the elementary chaotic existence…. 

Accordingly, in its essence and entire scope, death is disturbance, the breakup of all these relations in which humans stood originally and still ought to stand now. Death’s cause, therefore, is and can be none other than the sin that disturbs the right relation to God and breaks up life-embracing fellowship with God. In this sense sin not only results in death but also coincides with it; sin is death, death in a spiritual sense. Those who sin, by that token and at the same moment, put themselves in an adversarial relationship toward God, are dead to God and the things of God, have no pleasure in the knowledge of his ways, and in hostility and hatred turn away from him. And since this relation to God, this being created in his image and likeness, is not something extraneous and additional, a donum superadditum, but belongs to the essence of being human and bears a central character, the disturbance of this relationship will inevitably have a devastating impact on all the other relationships in which human beings stand—to themselves, to their fellow humans, to nature, to the angels, to the whole creation. Actually, in terms of its nature, at the very moment it was committed, sin should have resulted in a full, across-the-board death (Gen. 2:17), a return of the entire cosmos to its primeval chaotic condition.

But God intervened: he broke the power of sin and death. … He intervened first with his common grace to curb the power of sin and death, then with his special grace to break down and conquer that power. Not only is physical death postponed, and not only did God by various measures make human existence and development possible; but also Christ by his cross fundamentally achieved a victory over sin and death and brought life and immortality to light (Rom. 5:12ff.; 1 Cor. 15:45; 2 Tim. 1:10; Heb. 2:14; Rev. 1:18; 20:14), so that everyone who believes in him has eternal life and will never die (John 3:36; 5:24; 8:51–52; 11:25). Now it is this life and this immortality that in Holy Scripture stands in the foreground.

Herman Bavinck, Reformed Dogmatics, vol. 4, pp. 614-15.