One need not be "guilty" of anything in order to lose the supernatural virtue of faith. It can disappear through a kind of "atrophy". Once someone reaches the age of reason, an active assent of the intellect is required in order to sustain, as it were, the infused theological virtue of faith.
I hold the opinion that one need not actively sin against the faith in order to lose it. Pertinacious formal heresy uproots the faith precisely because one thereby loses the formal motive of faith, which is the authority of God revealing as proposed by the Church. But the formal motive of faith can also simply be absent, as in the case of someone raised by animists (after having been baptized) or someone raised among Protestants. Protestants lack the formal motive of faith. While they may hold certain beliefs materially (such as in the absolutely necessary articles--the Holy Trinity and the Incarnation), they do not believe them formally. Of course, some of this might be extremely vague in their minds, but as soon as any of the Protestant attitudes and mentality start infiltrating the mind, those squeeze out the true formal motive of faith and extinguish the supernatural virtue. God only knows exactly when that happens in the case of each individual.