On the flip side of this, Paul uses the symbol of baptism to convey some pretty important theological points pertinent only to the believer: he is washed, he is raised, he professes his faith, he shares the new life with Christ, probably a few other items too.
He does the same with circumcision.
For an outward Jew isn't one, nor is circumcision outward and physical. But an inward Jew is one, and circumcision is a matter of the heart, by the Spirit, not by the letter. His praise is not from man but from God. Rom 2:28-29
Look out for the dogs, look out for the evildoers, look out for those who mutilate the flesh. For we are the circumcision, who worship by the Spirit of God and glory in Christ Jesus and put no confidence in the flesh Pp 3:2-3
Obviously, circumcision is incapable of pictorially conveying these things. Even when Paul contrasts circumcision to baptism in Colossians, he's obviously forcing circumcision into an interpretive matrix determined by baptism (stripping away of flesh = washing away sin).
I don't see it as that obvious. Circumcision already is a purification rite, a separating of the flesh, which is what Paul stresses in Romans 8.
What's more, do pictures mean alot to you? How is getting dunked really pictorially like being buried? The two are pretty far apart. Only if this is a purification rite do the pictures close in on one another. And at that point they're two different purification rites, both emerging from OT practices.
Consequently, we can't say that the two are exactly the equivalent.
Hm, the equivalent of what? They're both purification rites. They're both signs of faith. What more correspondence do I need, again?
Why would God be unhappy with applying one sign of faith on an infant, when He commanded the other sign of faith on infants?
I think, moreover, the determining question for baptism for the earliest Christians was whether or not the person was a believer who makes his good confession before many witnesses, who signifies his death, burial, and resurrection with Christ in the watery grave, who depicts his forgiveness and washing away of sins while the living waters flow over him.
That's the thought of a Baptist, yes.
I think the determining question of the baptism of people who couldn't speak was the action itself signing the child to his God. Beyond that each and all of the needed pictures of baptism -- as if pictures were what God were about -- were in sprinkling (as the Didache permitted) or immersion, or even full submersion.
Not to mention the outpouring of the Spirit on him and his inclusion into the people of God.
As you know, the Spirit wasn't always poured out on people before baptism. And the infant's inclusion among the people of God is a given. The child of every believer is set apart to God.
By the way, I suppose that the Didache (about 70 A.D.) also teaches believers' baptism.
And so do infant baptists. But there's baptism for the minor children of believers as well.