With the acquisition of a new form there goes also the acquisition of the activity consequent upon that form; and therefore the baptised become immediately capable of spiritual actions, such as the reception of the other Sacraments. Also there is due to them a position suited to the spiritual life: that position is everlasting happiness: and therefore the baptised, if they die fresh from baptism, are immediately caught up into bliss: hence it is said that baptism opens the gate of heaven.
One and the same thing can be generated only once: therefore, as Baptism is a spiritual generation, one man is to be baptised only once. The infection that came through Adam defiles a man only once: hence Baptism, which is directed mainly against that infection, ought not to be repeated. Also, once a thing is consecrated, so long as it lasts, it ought not to be consecrated again, lest the consecration should appear to be of no avail: hence Baptism, as it is a consecration of the person baptised, ought not to be repeated.
4.58 : Of the Number of the Sacraments of the New Law
4.60 : Of Confirmation