In his discourse on “Justification by Faith alone” Jonathan Edwards thus defines justification: “A person is said to be justified when he is approved of God as free from the guilt of sin and its deserved punishment; and as having that righteousness belonging to him that entitles to the reward of life. That we should take the word in such a sense and understand it as the judge’s accepting a person as having both a negative and positive righteousness belonging to him, and looking on him therefore as not only quit or free from any obligation to punishment, but also as just and righteous, and so entitled to a positive reward, is not only most agreeable to the etymology and natural import of the word as used in Scripture.” He then shows us how it is, or why faith alone justifies. It is not on account of any virtue or goodness in faith, but as it unites us to Christ, and involves the acceptance of Him as our righteousness. Thus it is we are justified “by faith alone, without any manner of virtue or goodness of our own.”
The ground of justification is the righteousness of Christ imputed to the believer. “By that righteousness being imputed to us,” says Edwards, “is meant no other than this, that that righteousness of Christ is acceptable for us, and admitted instead of that perfect inherent righteousness that ought to be in ourselves: Christ’s perfect obedience shall be reckoned to our account, so that we shall have the benefit of it, as though we had performed it ourselves: and so we suppose that a title to eternal life is given us as the reward of righteousness…..The opposers of this doctrine suppose that there is an absurdity in it: they say that to suppose that God imputes Christ’s obedience to us, is to suppose that God is mistaken, and thinks that we performed that obedience that Christ performed. But why cannot that righteousness be reckoned to our account, and be accepted for us, without any such absurdity? Why is there any more absurdity in it, than in a merchant’s transferring debt or credit from one man’s account to another, when one man pays a price for another, so that it shall be accepted, as if that other had paid it? Why is there any more absurdity in supposing that Christ’s obedience is imputed to us, i.e., that it is accepted for us, and in our stead, and is reckoned to our account, as though we had suffered it. But why may not his obeying the law of God be as rationally reckoned to our account, as his suffering the penalty of the law?
–from Systematic Theology by Charles Hodge, quoting from Works of Jonathan Edwards