12 Therefore, just as sin came into the world through one man [Adam], and death through sin, and so death [both physical and spiritual] spread to all men because all sinned—
sin [literally—the sin] came [aorist—at one point in the past entered] into the world
Note that this phrase does not say “sins” (plural) entered the world but “sin” (singular). Sin, singular, commenced its course. Sin here represents inherited proclivity from the federal head, Adam, to violate God.
Sin in the singular is our inherited proclivity to sin.
From Adam’s first sin forward, all of his progeny have had a capacity to violate God’s norms. This capacity was henceforth branded by sin. Mankind has solidarity in this. Adam’s sin was spread to his posterity–“In Adam all die” (1 Co 15:22). All men have a federal head—Adam. Since he represents us in our capacity to sin, we have a federal head (Christ) who represents us in the forgiveness of sin.
The sin capacity here is not our primary capacity created by God but a secondary nature corrupted by the fall of Adam. It is a capacity to sin developed by the fall, a depraved nature transmitted to successive generations. Everyone except Christ has a natural inclination to sin or innate disposition toward sin. However, God gave believers a “divine nature” (2 Pe 1:4) by regeneration. This “nature” is a disposition toward God. There are therefore two capacities, one corrupted by the fall and the other produced by God toward God. In the case of the sin capacity, man through Adam is the sole author; in the case of the divine nature, God is the author.
While confessing his sin with Bathsheba, David attributed his sin to the wickedness of his heart or sinful disposition. He would never attribute his nature to God as the ultimate cause. He said, “In sin did my mother conceive me.” His sin capacity came from his mother, not his Maker.
This sin capacity does not render the conscience extinct but stupefies it. It cauterizes the conscience (1 Ti 4:2) and pollutes the conscience (Ti 1:15). The reasoning becomes “vain” or empty (Ro 1:21). This capacity therefore needs regeneration.
The will of the sin capacity is at enmity with God (Ro 8:6) and ultimately leads to hatred toward God (Ro 1:29). This in turn results in hardness of heart, aversion toward God, and obstinacy toward Him. Bondage in sin is the ultimate result of all of this (Ro 5:6; 2 Pe 2:14). Corruption of the capacity inclines the will to orient away from God, resulting in a status quo of guilt. The inclination to murder is as culpable as murder itself. Cause has the same predicate as effect. If a stream is corrupt, so is the fountain. God forgives both the inclination (state of the heart toward God) to sin and the sin itself.
Original sin is not the same as indwelling sin. The latter is what remains of original sin. The believer, however, carries a regenerate capacity (Ro 7:14-8:27). The sin capacity then does not need to be dominant and increasing in the believer, but can be in subjugation and in a diminishing state. Original sin rescinds in dynamics and does not have the same orientation after regeneration. In the unregenerate, the sin capacity rules the mind. In the believer, the sin capacity is in conflict with the Spirit and does not dwell peaceably in the Christian. Grace daily opposes this spiritual disease (Ro 6). The Christian needs to mortify it and continually die to it.