I'll comment on Bnonn's sequel:
The first thing I'd note is that Bnonn's follow-up argument is far more complicated that his original argument. Yet the advertised merit of his original argument was its simplicity. Well, it didn't take long to leave simplicity behind. In order to defend his original argument, the supporting argument becomes fairly complex. There's nothing necessarily wrong with that, of course. But that just means we're unlikely to win quick and easy victorious in longstanding theological debates.
"For instance, many pedobaptists argue that children of covenant members are themselves covenant members, or should at least be taken as such, until they are old enough to explicitly repudiate that membership."
That's certainly a popular contemporary formulation. But it's questionable whether that represents the traditional Presbyterian view:
"So pedobaptists take a “loose, implicit” approach to membership. Members of the community and members of the covenant seem broadly conterminous in their view."
That wasn't my argument. Indeed, my argument explicitly distinguished the two. I see them as overlapping categories.
Take this argument: kids that age are viewed as extensions of their parents. They aren't viewed as independent agents.
It's like belonging to a family. You were born into it.
"Conversely, credobaptists take a “tight, explicit” approach. There are many members of the community (children, unbelieving spouses, etc) who are not members of the covenant."
I could say the same thing in paedobaptist grounds, although I wouldn't use children to illustrate the point.
"For my own part, I think pedobaptism is the natural position to take if the new covenant is basically the same as the old, and we are applying its signs in the same way; and if baptism merely signifies membership in the covenant community, or a kind of “implicit” membership in the covenant."
i) No doubt there are crucial differences. But even the OT distinguished between physical circumcision and "circumcision of the heart."
ii) A basic problem I have with Bnonn's position is that it suffers from overrealized eschatology. One reason I distinguish between membership in the new covenant and membership in the new covenant community is because the church isn't heaven. In a fallen world, including the church, the vegetable garden inevitably has wheat and tares. You can't weed out all the tares. You can't even see all the tares.
That doesn't mean you should abstain from church discipline, when the occasion presents itself. But because the church is unavoidably a mixed community, unlike heaven (or the world to come), there is bound to be a distinction between membership in the covenant and membership in the covenant community.
Here below, the covenant community is a temporary and transitional organization. It's at the halfway mark on the journey.
So you have to strike a balance. If you are too strict about membership, you will exclude some born-again Christians. Exclude some of the elect. If you are too lax, the church will mirror the world.
"Christian baptism is modeled on the baptism of John; John’s baptism laid a foundation for and prefigured ours. But John’s baptism was a baptism of repentance (Acts 19:4). Those baptized were adults."
i) Isn't that appeal circular? If, by definition, John's baptism is a baptism of repentance; if, by definition, it was geared to adults, then by definition it's restricted to adults. It's like saying, "by definition, you can't be a married bachelor."
True, but tautologous. Like saying Alfa Romeo is an Italian sports car. No doubt. Does that mean all sports cars are Italian?
ii) Also, what was the age cut-off for John's baptism? Was it for adults, or did it include minors? Was the age of reason the threshold?
Was John baptizing 7-8-year-old kids? That doesn't seem to be the targeted demographic niche. But that makes it disanalogous with credobaptism.
"Baptism in Paul’s thinking signifies membership in the covenant because it signifies being made alive in Jesus—something reserved exclusively for covenant members.
Baptism is a symbol of regeneration. Thus it symbolizes covenant membership; not mere membership in the covenant community."
"Notice that baptism saves here. How? Not by removing spiritual dirt as water removes physical dirt, but by signifying our appeal to God through which are justified."
Bnonn prooftexted this understanding by quoting or citing Rom 6:1-4, 1 Cor 5:17, Eph 2:5, Col 2:9, 1 Pet 3:21.
i) Because the mode of baptism is controversial, translators sidestep that controversy by transliterating the Greek word. If they were to translate the word, that would prejudge the mode of baptism. And their restraint is a prudent policy.
ii) Apropos (i), the English word "baptism" denotes a Christian sacrament (or "ordinance"). In English usage, it's a technical term for the Christian rite of initiation. Sometimes it's used figuratively.
iii) Apropos (i-ii), this creates a risk of equivocation when citing baptismal texts from the NT. That's because, unlike the English word, the Greek word isn't a technical term for a Christian sacrament. It has more than one meaning. It can denote a Christian sacrament, or it can simply denote an action involving water. In addition, the Bible often uses aqueous theological metaphors.
When we read "baptism" (or "baptize") in the NT, we automatically associate that with a Christian sacrament. But that's conditioned by the connotations of the English word. The Greek word is less specialized and more polysemous.
In the Gospels and Acts, the Gospels may make it clear that the word is used for water baptism. It's not the word alone, but the word in conjunction with the setting, that makes it refer to baptism. Indeed, that would be implicit even in the absence of the word.
But in the epistles, those narrative clues are often lacking. So you can't just assume, without further ado, that the Greek word denotes a Christian sacrament rather than a picturesque theological metaphor.
iv) But even assuming, for the sake of argument, that all his prooftexts refer to the rite of baptism, they either prove too much or too little.
a) These passages are hortatory and idealistic. They urge the recipients of the letter to emulate what baptism signifies. But that implies a gap between the sign and what they actually are.
And that's a best-case scenario. That's for pious Christians who still fall short.
b) But, of course, Pauline churches also include Paul's opponents. They include heretics and apostates. What baptism signifies is in no sense true of them, even though they were baptized members of a local church planted by Paul.