My reading of the Bible indicates that when a person is about be baptized, he/she is confirming that he/she believes that Jesus is the son of God and believes in the name of the Father, the Son, and the Holy Spirit. But children don't know all this, so without their consciousness it seems inappropriate to baptize them. Why, then, do Catholics baptize children?
The New Testament indicates that the Apostoles baptized all members of some families, and we can assume in a family, there are kids :
"And I baptized also the household of Stephanus; besides, I know not whether I baptized any other." (1 Corinthians 1, 16)
(About Lydia) : "And when she was baptized, and her household" (Actes of the apostles 15, 16)
(Paul, to the keeper of the prison) : "Believe in the Lord Jesus, and thou shalt be saved, and thy house" ... "And he, taking them the same hour of the night, washed their stripes, and himself was baptized, and all his house immediately." (Actes of the apostles 15, 31.33)
Baptism is indeed sequel to conversion and faith (cf. Mark 16, 16; etc.) but as you see in the examples above, kids have been baptized on the basis of the faith of one parent. In the Catholic Church, for an infant to be baptized licitly, "there must be a founded hope that the infant will be brought up in the Catholic religion; if such hope is altogether lacking, the baptism is to be delayed according to the prescripts of particular law after the parents have been advised about the reason" (canon 868 §2).
Baptism brings other graces that it is worth children benefit :
- it makes them enter the Church
- it ereases original sin
- "According to the Apostle Paul, the believer enters through Baptism into communion with Christ's death, is buried with him, and rises with him (Romans 6,3-4 cf. Colossians 2,12). [...] The baptized have "put on Christ" (Galatians 3,27). Through the Holy Spirit, Baptism is a bath that purifies, justifies, and sanctifies (1 Corinthians 6,11 12,13)" (Catechism of the Catholic Church 1227)
Baptizing children is clearly letting them come to Christ. Let's not forbid them :
"And they brought to him young children, that he might touch them. And the disciples rebuked them that brought them. Whom when Jesus saw, he was much displeased, and saith to them: Suffer the little children to come unto me, and forbid them not; for of such is the kingdom of God" (Mark 10, 13-14)
Unfortunately, all the scriptural accounts of baptism describe only adult converts to Christianity. Scripture is almost silent when it comes to what they did for new children of those who were already baptized believers. This makes it difficult to come up with a scripture-only teaching on how to handle baptism.
While the bible doesn't explicitly say what the apostles taught on the practice of infant baptism, we may be able to figure it out by looking at historical records of the churches they created. The apostles were constantly visiting and sending letters to Christian churches all over the place (a lot of our new testament came from these letters) so we can say with almost certainty that they would have corrected any churches that were doing something as fundamental as baptism wrong.
When reading the historical records of early Christians, infant baptism was unanimously encouraged. For example, in his handbook "Apostolike Paradosis" Hippolytus in AD 215 said
" Baptize first the children, and if they can speak for themselves let them do so. Otherwise, let their parents or other relatives speak for them".
With so many historical sources all pointing to infant baptism, it is almost certain that the practice came from the apostles themselves. There is a list here of advocacy of infant baptism from early church fathers, to include:
“The Church received from the apostles the tradition of giving baptism even to infants. The apostles, to whom were committed the secrets of the divine sacraments, knew there are in everyone innate strains of [original] sin, which must be washed away through water and the Spirit” ~ Origen (Commentaries on Romans 5:9 [A.D. 248])