Roman Catholic teaching
The Roman Catholic Church is usually described as an institution that is sacerdotal, a term that comes from the Latin sacerdos, which means “priest.” Sacerdotalism holds that salvation is mediated through the functions of the priesthood, namely, the sacraments. Justification is a function of the sacerdotal operations of the church; that is, justification takes place primarily through the use of the sacraments, beginning with the sacrament of baptism. Rome says that the sacrament of baptism, among others, functions ex opere operato, which literally means, “through the working of the work.”
Protestants have understood this to mean that baptism works, as it were, automatically. If a person is baptized, that person is automatically placed in a state of grace, or in a state of justification. The Roman Catholic Church is quick to say it does not like to use the word “automatic,” because there has to be a certain predisposition in the recipient of baptism. At the very least, he or she must have no hostility toward the reception of the sacrament in order for it to function.
In any case, Rome has a high view of the efficacy of baptism to bring a person into a state of grace. This is because in the sacrament of baptism, grace is said to be “infused,” or poured into the soul. Protestants believe a person is justified when the righteousness of Christ is “imputed” or “credited” to his account.
Protestants likewise believe that our sins were imputed to Christ on the cross. That is, they were placed in His account, and He paid for them. So Protestants see in a “double imputation,” but Rome believes in “infusion,” which is the view that the righteousness of Christ is actually put into the believer, so that the person is actually righteous. The righteousness of Christ is not simply credited to the person’s account — it actually becomes the person’s possession.
The Council of Trent, defining its position in relation to the protests of the Reformers, used the terms cooperare et assentare, which means that believers must cooperate with, and assent to, the grace that is bestowed on them in baptism.
(The Sixth Session of the Council of Trent, Chapter V, emphasis added) “The Synod furthermore declares, that in adults, the beginning of the said Justification is to be derived from the prevenient grace of God, through Jesus Christ, that is to say, from His vocation, whereby, without any merits existing on their own parts, they are called; so that they, who by sins are alienated from God, may be disposed through His quickening and assisting grace, to convert themselves to their own justification by freely assenting to and co-operating with that said grace.“