No one enters the kingdom of God except he have sanctifying grace. God's first arrangement was to give sanctifying grace to every man in the moment when He created the man's rational soul. To speak as we should speak of a human scheme, this arrangement was defeated by Adam's sin. Consequently upon that sin, God arranged to give sanctifying grace, ordinarily, not in creation, but in baptism. Before baptism, the infant is devoid of sanctifying grace. That void is not a mere negation, it is a privation: for it is an absence of that which the child ought to have, if it is to answer to its Maker's design of leading all mankind to grace and the vision of Himself. Whence this privation? Through the sin of Adam, the head and representative of the human race, and therefore of that child. This privation of sanctifying grace, as traceable to the sin of the first parent, is original sin in that child.
Of God and His Creatures: 4.50