A common (though in my opinion needless) dispute within the Church is whether or not we should baptise infants. There are a number of ways to address this question. Some argue that, because there is evidence that infant baptism was practiced in the early Church, therefore it must necessarily be a good thing to do. Others would disagree, pointing out that since the Bible thoroughly equips us for every good work (2 Timothy 3:17), and yet nowhere mentions baptising infants, therefore baptising infants is, at best, futile.
But we can go one step further. Though the Bible neither says "baptise infants", nor does it say "do not baptise infants", it does tell us the purpose of baptism. Baptism is a declaration of faith. When a person came to Christ, they would seek to get baptised as soon as possible. One example is the Ethiopian Eunuch, to whom Philip preached the Gospel. The Eunuch accepted Philip's message, and as the two continued along the road, they came to some water. Immediately the Eunuch asked "what hinders me from being baptised?" Philip's response is "if you believe with all your heart, you may".
Now, the natural partner of an "if" is an "else". "Mummy, can I go and play with my toys?" "If you have finished your broccoli". "Boss, can I go home for the day?" "If you have finished today's work". When the word "if" is involved, the else doesn't even have to be explicitly stated. We instinctively know it is there. If the child has finished the broccoli, they may go to play with their toys. If they have not, they may not. If the worker has finished his work, he may go home. If he has not, he must stay.
In Acts 8:37, we find one such "if" command. If you believe with all your heart, you may be baptised. What is the natural "else"? The natural else is if you do not believe, you may not get baptised. From this, we can construct the following syllogism:
P1: Those who don't believe cannot be baptised.
P2: Infants cannot believe.
C: Infants cannot be baptised.
I believe the example of Philip and the Eunuch establishes the first premise. It's fairly simple logic. Ifs are paired with elses, the if is belief for baptism, the else is no baptism for non-belief. What about the second premise? This is where it gets complex. As with the age of accountability, I don't think it's possible to say "that child is definitely too young to believe". And in all honesty, I would prefer a non-believing child to get baptised than a child whose faith is genuine being denied baptism. After all, did Jesus not say "let the little children come to me, and do not hinder them"? (Luke 18:15-17). I fear that the ire of the Lord would be provoked if we forbade a believing child to be baptised.
However, though there is no definitive age at which a child can believe, is there not some standard age by which they can confess their belief? A newborn baby, though it can certainly scream, cannot declare, as the eunuch did, "I believe that Jesus Christ is the Son of God". Can a newborn profess Christ? Can a newborn understand Christ? A parents' faith, however admirable, cannot translate to the child. This is why we are to "train them up" (Proverbs 22:6). A parents' job is to teach their children to know the Lord, so that when they finally do come to faith, they may be baptised as believers, not as babes.
Thus, we can safely conclude that infant baptism is futile at best. That being said, I am hesitant to call it sin. To be sure, it is sin when it contradicts the Gospel (and indeed, a natural result of it is that many people are confused about the Gospel, as was I before I was saved), but what if the Gospel remains intact? In this case, I believe it can do no harm. Nevertheless, those who were baptised as infants should be encouraged to be baptised when they are finally able to confess faith as well.