Objection 1. It seems that what is done in the exorcism does not effect anything, but is a mere sign. For if a child die after the exorcisms, before being baptized, it is not saved. But the effects of what is done in the sacraments are ordained to the salvation of man; hence it is written (Mark 16:16): "He that believeth and is baptized shall be saved." Therefore what is done in the exorcism effects nothing, but is a mere sign.
Objection 2. Further, nothing is required for a sacrament of the New Law, but that it should be a sign and a cause, as stated above (Question 62, Article 1). If, therefore, the things done in the exorcism effect anything, it seems that each of them is a sacrament.
Objection 3. Further, just as the exorcism is ordained to Baptism, so if anything be effected in the exorcism, it is ordained to the effect of Baptism. But disposition must needs precede the perfect form: because form is not received save into matter already disposed. It would follow, therefore, that none could obtain the effect of Baptism unless he were previously exorcized; which is clearly false. Therefore what is done in the exorcisms has no effect.
Objection 4. Further, just as some things are done in the exorcism before Baptism, so are some things done after Baptism; for instance, the priest anoints the baptized on the top of the head. But what is done after Baptism seems to have no effect; for, if it had, the effect of Baptism would be imperfect. Therefore neither have those things an effect, which are done in exorcism before Baptism.
On the contrary, Augustine says (De Symbolo I): "Little children are breathed upon and exorcized, in order to expel from them the devil's hostile power, which deceived man." But the Church does nothing in vain. Therefore the effect of these breathings is that the power of the devils is expelled.
I answer that, Some say that the things done in the exorcism have no effect, but are mere signs. But this is clearly false; since in exorcizing, the Church uses words of command to cast out the devil's power, for instance, when she says: "Therefore, accursed devil, go out from him," etc.
Therefore we must say that they have some effect, but, other than that of Baptism. For Baptism gives man grace unto the full remission of sins. But those things that are done in the exorcism remove the twofold impediment against the reception of saving grace. Of these, one is the outward impediment, so far as the demons strive to hinder man's salvation. And this impediment is removed by the breathings, whereby the demon's power is cast out, as appears from the passage quoted from Augustine, i.e. as to the devil not placing obstacles against the reception of the sacrament. Nevertheless, the demon's power over man remains as to the stain of sin, and the debt of punishment, until sin be washed away by Baptism. And in this sense Cyprian says (Epist. lxxvi): "Know that the devil's evil power remains until the pouring of the saving water: but in Baptism he loses it all."
The other impediment is within, forasmuch as, from having contracted original sin, man's sense is closed to the perception of the mysteries of salvation. Hence Rabanus says (De Instit. Cleric. i) that "by means of the typifying spittle and the touch of the priest, the Divine wisdom and power brings salvation to the catechumen, that his nostrils being opened he may perceive the odor of the knowledge of God, that his ears be opened to hear the commandments of God, that his senses be opened in his inmost heart to respond."
Reply to Objection 1. What is done in the exorcism does not take away the sin for which man is punished after death; but only the impediments against his receiving the remission of sin through the sacrament. Wherefore exorcism avails a man nothing after death if he has not been baptized.
Praepositivus, however, says that children who die after being exorcized but before being baptized are subjected to lesser darkness. But this does not seem to be true: because that darkness consists in privation of the vision of God, which cannot be greater or lesser.
Reply to Objection 2. It is essential to a sacrament to produce its principal effect, which is grace that remits sin, or supplies some defect in man. But those things that are done in the exorcism do not effect this; they merely remove these impediments. Consequently, they are not sacraments but sacramentals.
Reply to Objection 3. The disposition that suffices for receiving the baptismal grace is the faith and intention, either of the one baptized, if it be an adult, or of the Church, if it be a child. But these things that are done in the exorcism, are directed to the removal of the impediments. And therefore one may receive the effect of Baptism without them.
Yet they are not to be omitted save in a case of necessity. And then, if the danger pass, they should be supplied, that uniformity in Baptism may be observed. Nor are they supplied to no purpose after Baptism: because, just as the effect of Baptism may be hindered before it is received, so can it be hindered after it has been received.
Reply to Objection 4. Of those things that are done after Baptism in respect of the person baptized, something is done which is not a mere sign, but produces an effect, for instance, the anointing on the top of the head, the effect of which is the preservation of baptismal grace. And there is something which has no effect, but is a mere sign, for instance, the baptized are given a white garment to signify the newness of life.
Source: Summa Theologica (New Advent)