Objection 1. It seems that the intention of receiving the sacrament of Baptism is not required on the part of the one baptized. For the one baptized is, as it were, "patient" in the sacrament. But an intention is required not on the part of the patient but on the part of the agent. Therefore it seems that the intention of receiving Baptism is not required on the part of the one baptized.
Objection 2. Further, if what is necessary for Baptism be omitted, the Baptism must be repeated; for instance, if the invocation of the Trinity be omitted, as stated above (66, 9, ad 3). But it does not seem that a man should be rebaptized through not having had the intention of receiving Baptism: else, since his intention cannot be proved, anyone might ask to be baptized again on account of his lack of intention. Therefore it seems that no intention is required on the part of the one baptized, in order that he receive the sacrament.
Objection 3. Further, Baptism is given as a remedy for original sin. But original sin is contracted without the intention of the person born. Therefore, seemingly, Baptism requires no intention on the part of the person baptized.
On the contrary, According to the Church's ritual, those who are to be baptized ask of the Church that they may receive Baptism: and thus they express their intention of receiving the sacrament.
I answer that, By Baptism a man dies to the old life of sin, and begins a certain newness of life, according to Romans 6:4: "We are buried together with" Christ "by Baptism into death; that, as Christ is risen from the dead... so we also may walk in newness of life." Consequently, just as, according to Augustine (Serm. cccli), he who has the use of free-will, must, in order to die to the old life, "will to repent of his former life"; so must he, of his own will, intend to lead a new life, the beginning of which is precisely the receiving of the sacrament. Therefore on the part of the one baptized, it is necessary for him to have the will or intention of receiving the sacrament.
Reply to Objection 1. When a man is justified by Baptism, his passiveness is not violent but voluntary: wherefore it is necessary for him to intend to receive that which is given him.
Reply to Objection 2. If an adult lack the intention of receiving the sacrament, he must be rebaptized. But if there be doubt about this, the form to be used should be: "If thou art not baptized, I baptize thee."
Reply to Objection 3. Baptism is a remedy not only against original, but also against actual sins, which are caused by our will and intention.
Source: Summa Theologica (New Advent)