Regarding infants and justification, here are some options we might consider.
1. Infants can be regenerate and united to Christ but not justified because they don’t have cognizant faith.
2. Infants who are regenerate have the seed of faith and are justified, though they haven’t yet exercised the seed of faith by believing.
3. There is no such thing as the seed of faith and regenerate infants are justified by union with Christ.
4. Infants who are regenerate mysteriously have cognizant faith and are resting upon Christ.
Some food for thought...
In Chapter 14 of the Westminster Confession of Faith, saving faith actually is distinguished from believing. The Confession teaches that through the grace of faith, the elect are enabled to believe to the saving of their souls (paragraph 14.1). See how it distinguishes the gift of faith and believing, which is the exercising of faith?
“By this faith, a Christian believes to be true whatsoever is revealed in the word…” (14.2.). The Confession does not teach that by this faith a Christian is enabled to have faith, for that would be unintelligible. Rather, the Confession teaches that by this faith – saving faith – God enables his elect to believe. In other words, by distinguishing faith and belief the Confession teaches that God effects the grace of faith by the Spirit of Christ in the hearts of His elect, whereby those with true faith, when confronted with the propositions of Scripture whereby they are understood, exercise this faith unto “obedience to the commands...” and many other “acts” of faith such as “accepting, receiving, and resting upon Christ alone for justification, sanctification, and eternal life…”
Notwithstanding, these “acts” of faith - even the principle act of faith - are not to be confused with the essence of faith, for as we have shown – by faith one believes, which in its principle act is accepting, receiving and resting upon Christ alone for the whole of salvation. So, when a person is regenerated he is granted the gift of faith. In time that faith will grow to believe in later years x,y and z, and not just the milk of the word, such as a, b, and c.
Because faith is distinguished from believing in the Confession’s chapter on saving faith, it is most reasonable to read 11.1 of the same Confession with that in mind. “Those who God effectually calls, He also freely justifies, not by infusing righteousness in to them…nor by imputing faith itself, the act of believing, or any other evangelical obedience to them, as their righteousness…” (Maybe read that again.)
In other words, it is most reasonable to interpret the Confession as not defining "faith itself" as “the act of believing" (lest it contradicts chapter 14!), but rather again distinguishing faith from the act of believing, just as it distinguishes faith from "evangelical obedience." In other words, the Confession teaches that God does not credit either (a) faith, (b) the act of believing, or (c) any other evangelical obedience to the sinner when he is pardoned, accepted and accounted as righteous.
Given such a distinction between faith and belief, it is easy to understand how a regenerate infant who is united to Christ can be justified by grace through faith alone – apart from understanding, believing and willfully embracing gospel propositions. (Position 2 above)
However, if justification is through faith alone and if the three “classic” elements of faith are necessary conditions for justification in infants, then regenerate infants (and those incapable of being called) cannot be pardoned for their sin! (Position 1 above)
Or else, regenerate infants can have cognizant faith, which I believe leads to all sorts of problems. (Position 4 above)
However, if infants can be justified, yet cannot have faith, then justification is by regeneration alone, apart from faith, when it comes to infants and those incapable of being called. That too is, I think, problematic. (Position 3 above)
At the very least, those who wish to maintain both that God may be merciful to infants and that justification is through a cognizant-faith alone have some theological reconciling to do. The simple solution is that those three elements pertain to belief and not to faith narrowly considered in seed form. After all, what about one who comes to Christ and then slips into a coma? He isn't believing in Christ (nor likely assenting, etc. to anything for that matter), but certainly he possesses the irrevocable gift of faith (though not being exercised). We have justifying faith when sleeping, but are we exercising faith at such times?
We must keep in mind that we are saved through faith so that our salvation might be of grace. Faith is the immediate result of regeneration, even prior to it being exercised by believing in Christ! Again, "by this faith one believes."