Objection 1. It seems that exorcism should not precede Baptism. For exorcism is ordained against energumens or those who are possessed. But not all are such like. Therefore exorcism should not precede Baptism.
Objection 2. Further, so long as man is a subject of sin, the devil has power over him, according to John 8:34: "Whosoever committeth sin is the servant of sin." But sin is taken away by Baptism. Therefore men should not be exorcized before Baptism.
Objection 3. Further, Holy water was introduced in order to ward off the power of the demons. Therefore exorcism was not needed as a further remedy.
On the contrary, Pope Celestine says (Epist. ad Episcop. Galliae): "Whether children or young people approach the sacrament of regeneration, they should not come to the fount of life before the unclean spirit has been expelled from them by the exorcisms and breathings of the clerics."
I answer that, Whoever purposes to do a work wisely, first removes the obstacles to his work; hence it is written (Jeremiah 4:3): "Break up anew your fallow ground and sow not upon thorns." Now the devil is the enemy of man's salvation, which man acquires by Baptism; and he has a certain power over man from the very fact that the latter is subject to original, or even actual, sin. Consequently it is fitting that before Baptism the demons should be cast out by exorcisms, lest they impede man's salvation. Which expulsion is signified by the (priest) breathing (upon the person to be baptized); while the blessing, with the imposition of hands, bars the way against the return of him who was cast out. Then the salt which is put in the mouth, and the anointing of the nose and ears with spittle, signify the receiving of doctrine, as to the ears; consent thereto as to the nose; and confession thereof, as to the mouth. And the anointing with oil signifies man's ability to fight against the demons.
Reply to Objection 1. The energumens are so-called from "laboring inwardly" under the outward operation of the devil. And though not all that approach Baptism are troubled by him in their bodies, yet all who are not baptized are subject to the power of the demons, at least on account of the guilt of original sin.
Reply to Objection 2. The power of the devil in so far as he hinders man from obtaining glory, is expelled from man by the baptismal ablution; but in so far as he hinders man from receiving the sacrament, his power is cast out by the exorcisms.
Reply to Objection 3. Holy water is used against the assaults of demons from without. But exorcisms are directed against those assaults of the demons which are from within. hence those who are exorcized are called energumens, as it were "laboring inwardly."
Or we may say that just as Penance is given as a further remedy against sin, because Baptism is not repeated; so Holy Water is given as a further remedy against the assaults of demons, because the baptismal exorcisms are not given a second time.
Source: Summa Theologica (New Advent)