I. The Doorway of Baptism.
- A) In Baptism, We Enter the Body of Christ, the Church.
Romans 6:3-5 says:
Or don't you know that all of us who were baptized into Christ Jesus were baptized into his death? We were therefore buried with him through baptism into death in order that, just as Christ was raised from the dead through the glory of the Father, we too may live a new life. If we have been united with him like this in his death, we will certainly also be united with him in his resurrection.
And more obviously, 1 Corinthians 12:13:
For we were all baptized by one Spirit into one body—whether Jews or Greeks, slave or free—and we were all given the one Spirit to drink.
- B) Children Are Called to Come to Christ.
Jesus said, "Let the little children come to me, and do not hinder them, for the kingdom of heaven belongs to such as these."
- C) Therefore, We Must Allow Children to be Baptized into Christ and His Church.
That's the first argument: simple enough. Here's the second:
II. The Saving Waters of Baptism.
- A) Sin and Impurity Keeps Us Out of Heaven.
Nothing impure will ever enter it, nor will anyone who does what is shameful or deceitful, but only those whose names are written in the Lamb's book of life.
2 Peter 2:4 notes that God did not spare even His angels when they sinned, casting them into Hell.
- B) Infants Are Born With Original Sin.
Romans 5:12-19 lays this out most clearly, as Paul contrasts Adam and Christ. I've highlighted the parts dealing with the imputation of Adam's sin:
Therefore, just as sin entered the world through one man, and death through sin, and in this way death came to all men, because all sinned— for before the law was given, sin was in the world. But sin is not taken into account when there is no law. Nevertheless, death reigned from the time of Adam to the time of Moses, even over those who did not sin by breaking a command, as did Adam, who was a pattern of the one to come.
But the gift is not like the trespass. For if the many died by the trespass of the one man, how much more did God's grace and the gift that came by the grace of the one man, Jesus Christ, overflow to the many! Again, the gift of God is not like the result of the one man's sin: The judgment followed one sin and brought condemnation, but the gift followed many trespasses and brought justification. For if, by the trespass of the one man, death reigned through that one man, how much more will those who receive God's abundant provision of grace and of the gift of righteousness reign in life through the one man, Jesus Christ.
Consequently, just as the result of one trespass was condemnation for all men, so also the result of one act of righteousness was justification that brings life for all men. For just as through the disobedience of the one man the many were made sinners, so also through the obedience of the one man the many will be made righteous.
Beyond this, virtually all Protestants teach the doctrine of Original Sin, so you can always just clarify the person's own beliefs on the issue.
- C) Baptism Washes Away Sin.
Paul, in giving his conversion story in Acts 22:16, quotes Ananias as saying:
"And now what are you waiting for? Get up, be baptized and wash your sins away, calling on his name."
Compare the contexts of Acts 9 and Acts 22. When Ananias says this, he's already laid hands on Paul, filling him with the Holy Spirit and giving him sight (see Acts 9:17-19). Paul believes, and then is Baptized, and it's in this second step, according to Ananias, that his sins are removed. Baptism isn't just a symbol, then, but actually removes sin.
St. Peter makes it perhaps even clearer in 1 Peter 3:20b-22, when he describes Noah's Ark as prefiguring the Church, and the floodwaters as prefiguring the waters of Baptism:
In it [the Ark] only a few people, eight in all, were saved through water, and this
water symbolizes baptism that now saves you also—not the removal of dirt from the body but the pledge of a good conscience toward God. It saves you by the resurrection of Jesus Christ, who has gone into heaven and is at God's right hand—with angels, authorities and powers in submission to him.
This is an issue on which the Bible is particularly clear.
- D) Therefore, We Must Allow Children to be Baptized to be Saved.
It's pretty basic. Kids are born dirty (through original sin). Baptism, and only Baptism, removes this spiritual dirt. Having this dirt removed is necessary to get into Heaven. To deny children Baptism, and just hope that they don't die before they reach the age of reason is to seriously jeopardize their spiritual well-being.
I can't promise that either of these arguments will be winners, particularly if someone approaches Baptism with the preconception that it's a symbol of our repentance. But they're worth a try. Also worth noting:
- Baptism as the new circumcision. Genesis 17:12-13 says circumcision is to happen on the eighth day (the first day that the blood can coagulate, suggesting God knew a bit more about medicine than modern athiests give Him credit for). It's an "everlasting covenant." In Christ, the Covenant is no longer performed physically, but spiritually, through Baptism (Colossians 2:11-12).
- Parents deciding for kids. This one is pretty easy. Some folks feel weird about deciding that their kid will be a Christian, instead of letting the kid have a personal conversion experience or decide for him/herself. That's moden nuttiness. As Joshua says in Joshua 24:15, "as for me and my household, we will serve the LORD."
Hope that helps!