I grew up Baptist and am now a Presbyterian. I will do my best to answer your questions as best I can.
I will not argue my point – “e.g., I’m right and you’re wrong,” but I will simply share our basis for infant baptism. Bottom line, I hope you can understand this point of view even though you may not agree with it.
Note: while most Christian denominations practice infant baptism, not all have the same meaning. I am sure that Chemnitz, (who is Lutheran) and I may share some of the same views, but then again, we may not.
Let me start by saying that both Presbyterians and Baptists agree on several issues regarding baptism. One, we would agree that baptism in and of itself, does not save. We are saved by grace through faith in Christ alone. Two, baptism is a sign of salvation. Three, we both believe in “believer’s baptism,” but we (Presbyterians) would define this as believers and their children.
Listed below are some questions and answers from a reformed faith perspective.
1. In the NT, when nonbelievers came to saving faith and joined the fellowship of believers, what happened? They were baptized.
2. Why? Baptism is the sign of salvation in the NT.
3. In the OT, when nonbelievers came to saving faith and joined the fellowship of believers, what happened? They were circumcised.
4. Why? Circumcism is the sign of salvation in the OT.
5. Who was circumcised? They and their household.
6. Did household include children? Yes.
7. In the OT, when believers had children, what happened? They were circumcised.
8. When were they circumcised - after they reached an “age of decision”? No, they were circumcised as infants.
9. Who came up with this idea? God commanded Abraham to do this.
Going back to question one, in the NT, when nonbelievers came to saving faith and joined the fellowship of believers, what happened? They and their whole household were baptized – given the sign of salvation, consistent with OT practices.
While the NT does not specifically mention the inclusion of infants in these households, the NT does not specifically exclude them either. Since the practice for the past 4000 years was to include infants and children, we would fully expect to see some statement in the NT that specifically excluded them -- if there was to be a difference in practice going forward. Instead, we find no such directive.
In our church, when nonbelievers come forward with a public profession of their faith and join our local fellowship, what happens? They and their whole household are baptized. When believers are blessed with a child, what happens? The infant is baptized. We believe both are in agreement with the Scriptures.
Again, I hope you can understand this point of view, though I am sure you will not necessarily agree.