These passages teach about baptism.
There was one of the Pharisees called Nicodemus, a leader of the Jews, who came to Jesus by night and said, 'Rabbi, we know that you have come from God as a teacher; for no one could perform the signs that you do unless God were with him.' Jesus answered: In all truth I tell you, no one can see the kingdom of God without being born from above. Nicodemus said, 'How can anyone who is already old be born? Is it possible to go back into the womb again and be born?' Jesus replied: In all truth I tell you, no one can enter the kingdom of God without being born through water and the Spirit; what is born of human nature is human; what is born of the Spirit is spirit.
(John 3:1-6)
Then he said, "The God of our ancestors has chosen you to know his will, to see the Upright One and hear his own voice speaking, because you are to be his witness before all humanity, testifying to what you have seen and heard. And now why delay? Hurry and be baptised and wash away your sins, calling on his name."
(Acts 22:14-16)
Christ himself died once and for all for sins, the upright for the sake of the guilty, to lead us to God. In the body he was put to death, in the spirit he was raised to life, and, in the spirit, he went to preach to the spirits in prison. They refused to believe long ago, while God patiently waited to receive them, in Noah's time when the ark was being built. In it only a few, that is eight souls, were saved through water. It is the baptism corresponding to this water which saves you now -- not the washing off of physical dirt but the pledge of a good conscience given to God through the resurrection of Jesus Christ, who has entered heaven and is at God's right hand, with angels, ruling forces and powers subject to him.
(1 Peter 3:18-22)
And he said to them, 'Go out to the whole world; proclaim the gospel to all creation. Whoever believes and is baptised will be saved; whoever does not believe will be condemned.
(Mark 16:15-16)
It happened that while Apollos was in Corinth, Paul made his way overland as far as Ephesus, where he found a number of disciples. When he asked, 'Did you receive the Holy Spirit when you became believers?' they answered, 'No, we were never even told there was such a thing as a Holy Spirit.' He asked, 'Then how were you baptised?' They replied, 'With John's baptism.' Paul said, 'John's baptism was a baptism of repentance; but he insisted that the people should believe in the one who was to come after him -- namely Jesus.' When they heard this, they were baptised in the name of the Lord Jesus, and the moment Paul had laid hands on them the Holy Spirit came down on them, and they began to speak with tongues and to prophesy. There were about twelve of these men in all.
(Acts 19:1-7)
These passages tell Christians that baptism effects the birth from above, washes away sins, saves us, and enables us to receive the Holy Spirit.
Infants need salvation, a new life in Christ, and the gift of the Holy Spirit hence they are baptised.
