Thirdmill's Dr. Joseph Nally Jr. answered it very well. This also deals with distinguishing infants' original and imputed sins, against those who said "infants can't be judged by God because they did not sin and that their death is payment enough for the ORIGINAL sin", as they left out the imputed sin.
I'm placing his answer here for preservation's sake:
How are original sin and imputed sin different? Please briefly compare and contrast.
Although it did not catch God off guard (1 Pet. 1:19-20) when Adam, the federal head of the universe (see below), fell in the Garden of Eden, there was a two-fold devastating effect: original and imputed sin.
Original sin means that all mankind enters into life with a fallen human nature that leaves us dead in trespasses and sin (Eph. 2:1-3). We're not sinners because we sin; we sin because we are sinners. We are all born already possessing the deadly cancer of sin. It's our very nature, part and parcel of our very being. We have amoral DNA and are morally ruined. Our hearts are absolutely corrupt (Jer. 17:9; Mark 7:21). We are totally depraved.
The entire human race inherited a sinful nature through Adam's original act of rebellion (Rom. 5:12-14). As such, we have sinful desires and tendencies and therefore it’s no surprise that original sin manifests itself in our sinful choices, actions, habits, feelings and thoughts (Jas. 1:14-15). It affects our entire disposition. It continually seeps from us as sweat does on a hot humid day. We naturally love sin and hate God (Rom. 8:7-8).
Imputed sin deals with the fact that the guilt of Adam is credited not just to Adam himself, but also to all humanity (Rom. 5:15, 18, 19). This doesn't mean we are personally guilty of Adam's sin (we're not) but that his sin was credited to us like into our account or ledger. In this way all of us, except Christ, who was born of a virgin (see below), are regarded as having sinned in the first Adam and share in his guilt. And before we think we could have done any better than Adam, we should remember that he, unlike us, was created "very good" (Gen. 1:31).
So, our standing before a holy God, the eternal Judge, is ruined, and, like Adam, we deserve the same judgment (1 Cor. 15:22). The wages of sin is death (Rom. 3:23; 6:23) both physically (Gen. 5:5, et. al.) and eternally (Rev. 20:11-15) and all of Adam's posterity bear the guilt and penalty of his original transgression.
When Adam sinned in the garden, he didn't just violate a commandment or two, he broke the entire law of God (Jas. 2:10). The result is we have polluted, sinful, and totally depraved natures because of Adam's sin (original sin), and we are considered as having sinned in Adam such that we are guilty of his act as well (imputed sin). Original sin affects our very character (we are totally depraved), whereas imputed sin affects our standing before holy God (we are condemned). Both place us under the judgment of Almighty God.
Since everything we are and do is so very sinful and doesn't and can't bring righteous glory to God and if we deserve to be eternally punished, then how can anyone be saved? Paul answers this question when he wrote the following:
But God demonstrates his own love for us in this: While we were still sinners, Christ died for us (Rom. 5:8, NIV).
For just as through the disobedience of the one man the many were made sinners, so also through the obedience of the one man the many will be made righteous (Rom. 5:19;).
The only remedy for original and imputed sin is the atoning work of Jesus Christ (cf. John 1:4; 10:9; 11:25; 14:6; Rom. 5:1-2; Eph. 2:18; Heb. 10:20; 1 John 5:20). Believers’ sins have been crucified with Jesus upon his cross (Col. 2:14; cf. Psa. 103:12). And in Christ, they possess imputed righteousness. The righteousness of Christ has been placed in their guilty accounts (Rom. 4:25; 2 Cor. 5:21).
Because of his loving, merciful, embracing grace, God takes a believer's filthy rags of his own righteousness (Isa. 64:6) and exchanges them for the absolute perfect righteousness of Christ (2 Cor. 5:21). This great exchange took place on the cross where Christ substituted his death for the believer's. As Peter wrote in 1 Peter 3:18, "For Christ died for sins once for all, the righteous for the unrighteous, to bring you to God. He was put to death in the body but made alive by the Spirit" (cf. 1 Pet. 2:24).
Whose flesh bore the just condemnation of the believer's sin? Christ Jesus'! (Isa. 53:5). Whose sins were condemned? The believer’s, not Christ who is sinless (2 Cor. 5:21). As Paul wrote, "And to the one who does not work but trusts him who justifies the ungodly, his faith is counted as righteousness" (Rom. 4:5).
When a believer places their faith in Jesus Christ alone, they are counted as righteous. The perfect righteousness earned by Jesus Christ is imputed to the believer. In turn, the believer's sins are imputed to Jesus Christ who made complete satisfaction for them by bearing the full wrath of God against his people on the cross (Rom. 5:1). This great exchange brings so great a salvation (Heb. 2:3).
Answer by Dr. Joseph R. Nally, Jr.
Dr. Joseph R. Nally, Jr., D.D., M.Div. is the Theological Editor at Third Millennium Ministries (Thirdmill).