Every child is born knowing that his or her Lord is Only One, Allah, regardless of what faith his or her parents may practice. But just because a child is born with this fitra, or natural belief, does not necessarily mean that the child will grow up to be what truly makes a person Muslim, which is ones iman.
It is a duty of every Muslim to keep his or her iman in check. This means one must constantly guard his or her iman and observe whether it has increased or decreased and for what causes. If it has decreased, one must work to increase it before it falls low enough to destroy the heart.