Objection 1. It seems that catechism should not precede Baptism. For by Baptism men are regenerated unto the spiritual life. But man begins to live before being taught. Therefore man should not be catechized, i.e. taught, before being baptized.
Objection 2. Further, Baptism is given not only to adults, but also to children, who are not capable of being taught, since they have not the use of reason. Therefore it is absurd to catechize them.
Objection 3. Further, a man, when catechized, confesses his faith. Now a child cannot confess its faith by itself, nor can anyone else in its stead; both because no one can bind another to do anything; and because one cannot know whether the child, having come to the right age, will give its assent to faith. Therefore catechism should not precede Baptism.
On the contrary, Rabanus says (De Instit. Cleric. i): "Before Baptism man should be prepared by catechism, in order that the catechumen may receive the rudiments of faith."
I answer that, As stated above (Question 70, Article 1), Baptism is the Sacrament of Faith: since it is a profession of the Christian faith. Now in order that a man receive the faith, he must be instructed therein, according to Romans 10:14: "How shall they believe Him, of Whom they have not heard? And how shall they hear without a preacher?" And therefore it is fitting that catechism should precede Baptism. Hence when our Lord bade His disciples to baptize, He made teaching to precede Baptism, saying: "Go ye... and teach all nations, baptizing them," etc.
Reply to Objection 1. The life of grace unto which a man is regenerated, presupposes the life of the rational nature, in which man is capable of receiving instruction.
Reply to Objection 2. Just as Mother Church, as stated above (69, 6, ad 3), lends children another's feet that they may come, and another's heart that they may believe, so, too, she lends them another's ears, that they may hear, and another's mind, that through others they may be taught. And therefore, as they are to be baptized, on the same grounds they are to be instructed.
Reply to Objection 3. He who answers in the child's stead: "I do believe," does not foretell that the child will believe when it comes to the right age, else he would say: "He will believe"; but in the child's stead he professes the Church's faith which is communicated to that child, the sacrament of which faith is bestowed on it, and to which faith he is bound by another. For there is nothing unfitting in a person being bound by another in things necessary for salvation. In like manner the sponsor, in answering for the child, promises to use his endeavors that the child may believe. This, however, would not be sufficient in the case of adults having the use of reason.
Source: Summa Theologica (New Advent)