I was baptized Catholic as an infant, and as an adult Catholic I am required to believe that that baptism was sacramentally valid, even though, as an infant, I had no capacity for personal belief in Christ. This fact of the infant psychology—the incapacity to have faith—is the source of the longstanding protestant belief that the Catholic practice of infant baptism is a grave error and ultimately invalid. After all, Christ taught that "Whoever believes and is baptized will be saved", which suggests that personal belief is a prerequisite for baptism and salvation.
In Catholicism, we hold that the faith of the parents, and the godparents, "stands in" for the faith that the infant is as yet incapable of having, and that this is sufficient because the infant cannot place any impediment to faith. I always felt that this got me 80% of the way there, but was a bit dubious. My belief that the Catholic Church is the church founded by Christ and therefore is guarded from doctrinal error by the mysterious workings of the Holy Spirit supplied the missing 20%.
The other day, I had a revelation that finally sealed the deal unequivocally. I observed that for children the parent's faith does in fact stand in. Not in any mysterious sacramental way, but in a very natural psychological way. Consider, for instance, the way that every child below a certain age whose parents tell him Santa Claus is real believes that Santa Claus is real. Eventually, he will learn that the Santa Claus thing is a fantasy, and that his parents don't really believe in it, at which point he will drop that belief altogether. In a similar way, whatever parents tell their child regarding God, he will believe. He may hear that God, too, is a fantasy, but seeing that his parents really believe in God and don't consider it a fantasy, he will retain his faith.
Until, of course, he develops the capacity to think for himself. Sometime in his teenage years, perhaps earlier if he is precocious, he will have the psychological capacity and the desire to decide on his own about God. He will either affirm for himself the faith that his parents gave him, a process sacramentalized by the Church in the Sacrament of Confirmation, or he will reject it.
At any rate, I now understand that infant baptism is entirely legitimate, because for little children there is no distinction between what their parents believe and what they believe. Their parents' faith is their own until they are old enough. And as Christ says, "Let the little children come to me, and do not hinder them, for the kingdom of heaven belongs to such as these."