“Seminary word” of the day: Imputation
To "impute" is "ascribe to" something. It is from Latin imputare: in (which means, well, "in") and put (which means "to settle an account"). In theology, imputation usually refers to the doctrine of Justification. Charles Hodge writes, "The righteousness of Christ is imputed to the believer for his justification. . . . To impute is to ascribe to, to reckon to, to lay to one’s charge."
Protestants believe that this includes the imputation of Christ's righteousness (a kind of credit placed in the believer's "account"). Imputed righteousness is the declaration of a believer's righteousness. This righteousness is not the believer's own, however. Roman Catholic theology emphasizes that God justifies in accord with an infused righteousness merited by Christ. Hodge responds that, "It seems unnecessary to remark that this does not, and cannot mean that the righteousness of Christ is infused into the believer, or in any way so imparted to him as to change, or constitute his moral character. Imputation never changes the inward, subjective state of the person to whom the imputation is made."
Why is such imputation necessary? Because sin, it seems, is also imputed. Charles Ryrie writes that, "The debate centers around the meaning of Romans 5:12 and especially the last words in that verse—“all sinned.” Do they mean that all are sinners (which is essentially saying that all have a sin nature) or do they mean that in some way all mankind sinned when Adam sinned? If the latter, then this is imputed sin."
Imputed sin, however it is understood, along with man’s personal sin is why, without imputed righteousness, no one would enter heaven.
For our sake he made him to be sin who knew no sin, so that in him we might become the righteousness of God.
(2 Cor. 5:21)