A relevant passage to the discussion below from the chapter in my book, Forming Intentional Disciples, on sacramental grace and personal faith/disposition:
As I have noted before, Blessed John Paul II wrote about baptized Christians who were “still without any explicit personal attachment to Jesus Christ; they only have the capacity to believe placed within them by Baptism and the presence of the Holy Spirit.” The pope was reflecting a long-standing theological distinction that we don’t usually make in English.
We often use the single English word faith to denote what is covered by two different Latin terms. The first Latin term, virtus fidei, means “the virtue of faith,” which is the power or capacity to believe but not the act of faith itself. Virtus fidei is the sort of faith that Blessed John Paul II was referring to when he wrote of the “capacity to believe” placed within us by baptism that can exist without explicit personal attachment to Jesus Christ.
The Church uses a different term, actus fidei, for the explicit, personal act of faith that is at the heart of discipleship.
"The Catholic tradition holds that it is the virtue of faith that is bestowed in baptism. For that virtue to become a personal act of faith, it must be activated freely, explicitly, however minimally…. It is that personal act of faith, however minimal, and always under the grace of God, that transforms the human being from one who can be a believer into one who is a believer…. It is that act of faith that is required for right sacramental intention.
The virtus fidei, the “capacity to believe” bestowed upon an infant at baptism, must become actus fidei, explicit personal faith, for a teen or adult to receive a sacrament fruitfully.
 Catechesi Tradendae, 19, my emphasis.
 Marriage and the Catholic Church, p. 54-55.