Why do you baptize infants and how do you justify it with Scripture?
Mark 2:1-12 records the account of a paralytic who was brought to Jesus by his friends. Upon reaching the home where Jesus was teaching, they lowered him through the roof, since there was no way to enter the home through the crowd. Verse 5 records, "And when Jesus saw their faith, he said to the paralytic, 'My son, your sins are forgiven.'" Notice that Jesus, upon seeing their faith, healed the paralytic. It was not, at least at that point, as a result of the paralytic's faith.
Similarly, this is what we do when we bring a child to be baptized. We come in faith on behalf of the child. The parents and godparents take the responsibility upon themselves to raise the child in the faith.
Baptism begins a journey, rather than the culmination of a journey of faith. As Frank Wilson in Faith and Practice explains, "It is the door into the Kingdom of God, the agency of new birth, the beginning of new life.
It unites us with Christ, makes us members of His Church, imparts forgiveness of sins, and equips us to receive the further gifts of God's grace."
In baptism, we are marked and sealed as Christ's own forever. We are set apart.
The guilt of original sin, that disobedience of Adam that separated all from God, is washed away. From that moment onward we begin a new life in Christ.
The journey starts, and for children, it entails learning the faith from those who pledged and promised before God and His Church to teach it to them. They are guarded by the parents and godparents until the time when they take ownership of this themselves.
This is a part of the Christian life. All Christians have a choice daily to follow Jesus by making decisions that draw us closer to Him. We do not arrive in our faith.
In the same manner that our calendars mark time as BC to AD, with the defining moment of the entrance of Jesus into the world, so too our personal lives have BC to AD with the defining moment being baptism. From that moment onward we are called to grow daily in holiness, or sanctity, until the day we arrive "face to face" in God's presence.