I am a Protestant. As such, I contend that salvation comes about by the agency of divine grace through the means of faith. Such a position is not (typically) challenged among sincere Protestants qua Protestants. Nevertheless, the simplistic formula does betray a few tensions. Not least of these tensions is the question of how infants or the mentally disabled (for any number of reasons) can be “saved through faith.”
Low-church Protestants, among whom one would count Baptists, most non-denominational churches, etc., emphasize faith as an individual’s individual grasping of Christ for salvation. This “grasping” is usually demarcated so as to discount “insincere” or “inauthentic” expressions of faith, such that one who ostensibly has faith in Christ but whose life does not reflect this should have their faith questioned. Nevertheless, the emphasis falls on the individual coming to terms with his individual relationship to Christ.
Insofar as this describes the typical process of coming-to-faith, the construction has no intrinsic difficulties. The Catholic or the Orthodox would push back against such a simplistic definition of faith, but not because the definition itself is unreasonable. However, one has to wonder how this standard Protestant definition of faith to salvation can be reckoned with the death of an infant or child or of an adult who has lost cognitive capacity, whether through a developmental disability or through a degenerative disease such as Alzheimer’s. It seems that neither the young one nor the adult, due to biological capacities, are able to either express or hold faith in a manner equivalent to the neurotypical person.
A typical answer, when asked about the fate of a child who dies, is that the young child is below “the age of accountability,” or the age at which a child (1) understands sin and (2) therefore is responsible for her sin. Let’s ignore for the moment the fact that this line of argumentation is completely ad hoc. Let’s also ignore the fact that this doctrine tends to leave small children as “less-than” in the body of the church. In fact, the practice of Baptist churches tends to underline this perception: young children can be withheld from baptism or communion until they reach a certain age and can “intelligently express” faith. High-church traditions avoid this unnecessary dichotomy by baptizing children at birth, whether for salvation (Catholic, Orthodox) or for covenant-marking (Presbyterian).
The question remains, however, as to how it can be held consistently that salvation comes by faith but children or the intellectually disabled are saved despite not having an appropriate intellectual capacity for “faith-expression” or “faith-formation.” It would seem that this only becomes a problem when faith-to-salvation becomes formalized as the means of salvation for individuals. Once faith-to-salvation becomes the formula for salvation, one must use either ad hoc formulations (the age of accountability, for example), distinct church praxis (paedobaptism), or intrinsic inconsistency to juxtapose the death of the infant and the disabled with the mercy of God. Concerning the last point, one can say that God saves children mysteriously despite their sin and despite their non-expression of faith. Similarly, one can say that God saves the mentally disabled through special grace that we cannot ascertain.
However, perhaps, if one does not begin to see faith-to-salvation as the formula, then the door remains open for God to act in mercy even towards those who have not expressed faith, without endangering the faith-to-salvation formula itself through inconsistency. That is to say, by opening the means of salvation to something other than faith-to-salvation and special cases, one can say that God saves the infant or the mentally disabled without thereby endangering God’s justice (howsoever defined). By saying that God is in his essence “compassionate and gracious, slow to anger, abounding in love and faithfulness,” (Ex. 34.6), one can proclaim that God shows unqualified mercy to all, especially those who could not comprehend the message of faith and repentance.