The earliest explanation for the sin of Adam and the guilt of all his descendants was the realistic theory which states that human nature constitutes both generically and numerically a single unit. The same substance which acted in Adam and Eve, having been communicated to us, their act was as truly and properly our act, being the act of our reason and will, as it was their act. It is imputed to us therefore not as his, but as our own. This means humanity literally sinned in Adam, and consequently the guilt of that sin is our personal guilt and the consequent corruption of nature is the effect of our own voluntary act. “The total guilt of the first sin, thus committed by the entire race in Adam, is imputed to each individual of the race, because of the indivisibility of guilt.” This means that each individual nature is guilty and corrupt for the whole of the first sin or “offense” against God because even though the common nature is divisible by propagation, the offense and the guilt are not divisible.
The Scriptures teach that the sin of our first parents constituted their posterity sinners (Rom 5:19), so that Adam’s sin is imputed, reckoned, or charged to every member of the race of which he was the germ and head (Rom 5:16). Humanity is born depraved and subject to God’s disciplinary afflictions because of Adam’s sin (Rom 5:12; Eph 2:3). The realistic view must deal with two question – first, how are humans responsible for a depraved nature that they did not personally or consciously originate; and, secondly, how can God justly charge to a humans account the sin of Adam. The realistic view answers this by saying that Adam’s sin is the cause and ground of the depravity because Adam and his posterity are one, and, by virtue of that unity, the sin of Adam is the sin of the race (Rom 5:12; 1 Cor 15:22). This invisible substance or nature, that is capable of being transformed into multitudes of self-conscious individuals, while in Adam and not distributed yet, had no distinct self-consiousness of its own. Everything that constitutes this substance, namely, Adam and the human nature in him, is active and responsible for all that is done by this unity. The individual Adam and Eve were no more guilty of this first act and of the whole of it than their descendants are; and their descendants are as guilty as they.
Shedd argues that all were united in Adam when he disobeyed, yet all were not in Christ when He obeyed. Everyone is propagated from Adam and inherit his sin; yet no person is propagated from Christ or inherits His righteousness. In Adam, our apostasy is universal. In Christ, it is particular and by election. “As the unmerited imputation of Christ’s obedience conveys the total undivided merit of this obedience to each and every believer, so the merited imputation of Adam’s disobedience conveys the total undivided guilt of this disobedience to each and every individual of the posterity.” Humanity must sin in Adam in order to be justly punished for Adam’s sin, and participation requires union with Adam. The imputation of Adam’s sin rests upon a different kind of union from that upon which the imputation of Christ’s righteousness rests.
 Louis Berkhof, Systematic Theology (Grand Rapids: Wm. B. Eerdmans, 1938), 241.
 Hodge, Systematic Theology, 193.
 Shedd, Dogmatic Theology, 560.
 Strong, Systematic Theology, 593.
 Shedd, Dogmatic Theology, 564.
 Ibid, 560.
 Ibid, 562.