In the opening paragraph of his discussion of Lord's Day 3, for which his exposition centers on Romans 5:12, Ames writes
"The apostle's counsel in this place is to illustrate the doctrine that had previously been handed down concerning justification through Jesus Christ, which he established for the purpose of a comparison of [i]the similarity between this grace of our Lord Jesus Christ and the sin of our first parent, Adam. The comparison hinges on the efficacy and the effects of both.[/i] Verse 12 contains a proposition of comparison. The result is explained afterwards by a parenthesis. In this proposition, Adam is placed as the cause of a [i]double effect: the introduction of sin and of death.[/i]" (p. 16, A Sketch of the Christian's Catechism, William Ames)Ames is making a very important point here, and one which is central to our understanding of covenant relations between God and man. The aim here is to connect to Paul's main thrust in Romans 5:12-21 - namely the likeness of the two Adams. Note in this regard the [i]double[/i] detriment that Ames discusses - SIN and DEATH - that arises in all people who are in Adam, born condemned (conceived condemned and under a sentence of death) and also tainted with sin such that sin is the only outcome of any act. Not only are people 1) judicially guilty, through the imputation of a guilty verdict passed on their covenant head, but they also 2) can do nothing but sin in their depraved nature. Adam's sin purchased (in a sense) both of these 'benefits' for all those who are in Adam (which is all humanity).
This two-fold effect of Adam is matched by a two-fold benefit of Christ, and this observation is, I believe, a critical one for us today in the church to understand. Christ by his life, death and resurrection benefits those who are in him with a two-fold grace... 1) justification - the judgment of them as FULLY righteous - declared to be without guilt and declared to owe no penalty for sin; and 2) sanctification - the process of becoming what they are, righteous and holy, by means of the blessing of the indwelling holy spirit. Christ purchased both of these benefits for all those who are in Him (which is all His people, the elect of God).
The two-fold detriment of Adam is matched to perfection by the two-fold grace of Christ. In order to understand the latter, it is utterly critical that one understand the former!