Objection 1. It seems that the Church observes an unsuitable rite in baptizing. For as Chrysostom (Chromatius, in Matth. 3:15) says: "The waters of Baptism would never avail to purge the sins of them that believe, had they not been hallowed by the touch of our Lord's body." Now this took place at Christ's Baptism, which is commemorated in the Feast of the Epiphany. Therefore solemn Baptism should be celebrated at the Feast of the Epiphany rather than on the eves of Easter and Whitsunday.
Objection 2. Further, it seems that several matters should not be used in the same sacrament. But water is used for washing in Baptism. Therefore it is unfitting that the person baptized should be anointed thrice with holy oil first on the breast, and then between the shoulders, and a third time with chrism on the top of the head.
Objection 3. Further, "in Christ Jesus... there is neither male nor female" (Galatians 3:23)... "neither Barbarian nor Scythian" (Colossians 3:11), nor, in like manner, any other such like distinctions. Much less, therefore can a difference of clothing have any efficacy in the Faith of Christ. It is consequently unfitting to bestow a white garment on those who have been baptized.
Objection 4. Further, Baptism can be celebrated without such like ceremonies. Therefore it seems that those mentioned above are superfluous; and consequently that they are unsuitably inserted by the Church in the baptismal rite.
On the contrary, The Church is ruled by the Holy Ghost, Who does nothing inordinate.
I answer that, In the sacrament of Baptism something is done which is essential to the sacrament, and something which belongs to a certain solemnity of the sacrament. Essential indeed, to the sacrament are both the form which designates the principal cause of the sacrament; and the minister who is the instrumental cause; and the use of the matter, namely, washing with water, which designates the principal sacramental effect. But all the other things which the Church observes in the baptismal rite, belong rather to a certain solemnity of the sacrament.
And these, indeed, are used in conjunction with the sacrament for three reasons. First, in order to arouse the devotion of the faithful, and their reverence for the sacrament. For if there were nothing done but a mere washing with water, without any solemnity, some might easily think it to be an ordinary washing.
Secondly, for the instruction of the faithful. Because simple and unlettered folk need to be taught by some sensible signs, for instance, pictures and the like. And in this way by means of the sacramental ceremonies they are either instructed, or urged to seek the signification of such like sensible signs. And consequently, since, besides the principal sacramental effect, other things should be known about Baptism, it was fitting that these also should be represented by some outward signs.
Thirdly, because the power of the devil is restrained, by prayers, blessings, and the like, from hindering the sacramental effect.
Reply to Objection 1. Christ was baptized on the Epiphany with the Baptism of John, as stated above (Question 39, Article 2), with which baptism, indeed, the faithful are not baptized, rather are they baptized with Christ's Baptism. This has its efficacy from the Passion of Christ, according to Romans 6:3: "We who are baptized in Christ Jesus, are baptized in His death"; and in the Holy Ghost, according to John 3:5: "Unless a man be born again of water and the Holy Ghost." Therefore it is that solemn Baptism is held in the Church, both on Easter Eve, when we commemorate our Lord's burial and resurrection; for which reason our Lord gave His disciples the commandment concerning Baptism as related by Matthew (28:19): and on Whitsun-eve, when the celebration of the Feast of the Holy Ghost begins; for which reason the apostles are said to have baptized three thousand on the very day of Pentecost when they had received the Holy Ghost.
Reply to Objection 2. The use of water in Baptism is part of the substance of the sacrament; but the use of oil or chrism is part of the solemnity. For the candidate is first of all anointed with Holy oil on the breast and between the shoulders, as "one who wrestles for God," to use Ambrose's expression (De Sacram. i): thus are prize-fighters wont to besmear themselves with oil. Or, as Innocent III says in a decretal on the Holy Unction: "The candidate is anointed on the breast, in order to receive the gift of the Holy Ghost, to cast off error and ignorance, and to acknowledge the true faith, since 'the just man liveth by faith'; while he is anointed between the shoulders, that he may be clothed with the grace of the Holy Ghost, lay aside indifference and sloth, and become active in good works; so that the sacrament of faith may purify the thoughts of his heart, and strengthen his shoulders for the burden of labor." But after Baptism, as Rabanus says (De Sacram. iii), "he is forthwith anointed on the head by the priest with Holy Chrism, who proceeds at once to offer up a prayer that the neophyte may have a share in Christ's kingdom, and be called a Christian after Christ." Or, as Ambrose says (De Sacram. iii), his head is anointed, because "the senses of a wise man are in his head" (Ecclesiastes 2:14): to wit, that he may "be ready to satisfy everyone that asketh" him to give "a reason of his faith" (cf. 1 Peter 3:15; Innocent III, Decretal on Holy Unction).
Reply to Objection 3. This white garment is given, not as though it were unlawful for the neophyte to use others: but as a sign of the glorious resurrection, unto which men are born again by Baptism; and in order to designate the purity of life, to which he will be bound after being baptized, according to Romans 6:4: "That we may walk in newness of life."
Reply to Objection 4. Although those things that belong to the solemnity of a sacrament are not essential to it, yet are they not superfluous, since they pertain to the sacrament's wellbeing, as stated above.
Source: Summa Theologica (New Advent)