I reject the notion that one has to commit an active sin against the faith to lose supernatural faith.  When an infant is baptized, that's a special case where the supernatural virtue of faith is merely infused.  There can be no merely-infused supernatural virtue of faith in adults, i.e. those who have the use of reason.  Consequently, when a child reaches the use of reason, if the supernatural faith isn't confirmed with actual acts of faith, the virtue is lost, and without supernatural faith there can be no supernatural charity.  To think that someone can grow up, reach the age of reason, and then be an atheist, say having been raised as such, and the stay in a state of persevering in supernatural faith.

Sean,

I think Ladislaus was correct about personal faith in Christ being necessary upon reaching maturity.

It's not a question of forfeit, but rather God willing salvation be through faith in His Son, for all capable of knowing - I do not mean capable by or under external circuмstance, but capable of having the "hardware" or "software" required to will and know.

A baptized person, upon  reaching maturity, will be confronted with moral choices. In the hypothetical case that a person does not sin before the approach of death, before departing this earth God will provide such a person with knowledge of, and faith in, Christ - either through provision of a preacher, or by internal inspiration of the Holy Ghost. That is St. Thomas's sound teaching.

Of course, if a person sins at some point after reaching maturity, faith in Christ, with repentance, is the sine qua non of dying in a state of grace.

DR

Sean, you keep going back to the notion that a sin has to be committed against faith to lose supernatural faith.  Normally yes, for an adult, but I hold infants to be a special case.

Even the "Rewarder God" folks held that explicit faith in a God who rewards the good and punishes the wicked is a sine qua non for supernatural faith ... for adults.  I hold, with St. Thomas and others, that explicit knowledge of Christ and the Holy Trinity are also essential.  In fact, this was held unanimously until the discovery of the New World, when a Franciscan and some Jesuits floated Rewarder God theory in response to finding all those unevangelized masses.  In any case, until Bergoglio said it some years ago, no Catholic ever entertained the notion that an atheist, someone who had explicit faith in nothing, could be saved.  So, if the infant grows up to acquire the use of reason but does not have any explicit belief in anything, that supernatural virtue of faith fades away, like the seed sown on the rocky ground where it has nothing to take root in.  As to why God may have allowed this, we can only speculate, but St. Thomas holds that this ignorance, if invincible, is itself not a sin, but God would allow this to happen on account of other sins.  It can also be God's Mercy, as perhaps He knows that the person would end up rejecting the faith and therefore meriting a worse eternal fate.  We don't know.

Based on how you're asking the question, in your scenario, let's imagine an infant who's secretly baptized by some overzealous individual and ends up being raised by atheists.  He reaches the age of reason, and then dies at the age of 15 without having actively rejected the faith or committed any other grave sin.  Would that person be saved?  To say yes would be to say that atheists can be saved without explicit faith in anything ... which no Catholic thinker or theologian has ever held prior to Bergoglio's utterance.  You're saying that individuals can have some infused supernatural virtue of faith without any knowledge or awareness of it.  You'd basically be promoting a variation of Rahner's Anonymous Christian theory and agreeing with Bergoglio that atheists can be saved.

Lad-

please explain how grace is lost without sin (or conversely, how those in the state of grace are damned).

Lad,

In your opinion, can the state of grace be lost without mortal sin?

I've already answered this question.  Did you not read what I wrote?  Yes, it can be, in the unique case of an infant where the supernatural virtue of faith is merely infused.  Then without supernatural faith, there can be no supernatural virtue.  You're framing your question from the mindset of a normal adult in a state of sanctifying grace.  That is completely different from the state of an infant who has merely infused supernatural virtues.  You continue to be fixated on this notion because your mind can't grasp anything outside the normal paradigm for adults.

Now answer my question about whether you think an atheist in the above scenario can be saved.

No one who has reached the age of reason is in a moral vacuum. Every act is either virtuous or sinful. If it is gravely sinful, grace is lost. This child, through baptism, has grace. If he does not commit mortal sin, the grace is never lost.
It is right there in the very first book of Holy Scripture: "Before man is good and evil. That which he chooses shall be given him".