Download the Regulative Principle App (and the Irregulative Principle)

The Regulative Principle of Worship (RP) is, from Wikipedia:

The regulative principle of worship is a Christian doctrine, held by some Calvinists and Anabaptists, that God commands churches to conduct public services of worship using certain distinct elements affirmatively found in scripture, and conversely, that God prohibits any and all other practices in public worship. The doctrine further determines these affirmed elements to be those set forth in scripture by express commands or examples, or if not expressed, those which are implied logically by good and necessary consequence.

It is also found the in great reformed confessions, for example, the WCF and LBCF 1689 have the same language:

The light of nature shews that there is a God, who hath lordship and sovereignty over all; is just, good and doth good unto all; and is therefore to be feared, loved, praised, called upon, trusted in, and served, with all the heart and all the soul, and with all the might. But the acceptable way of worshipping the true God, is instituted by himself, and so limited by his own revealed will, that he may not be worshipped according to the imagination and devices of men, nor the suggestions of Satan, under any visible representations, or any other way not prescribed in the Holy Scriptures.

Do not bother trying to prove, from scripture, that the RP applies to the New Testament church, because you can't. The proof texts you find in some renderings of the confessions often (usually?) are not proof texts at all, but as one pastor once correctly characterized them, "starting points." If you expect the proof texts in, say, The Westminster Confession of Faith with  Scriptural Proofs actually live up to the billing, you will be, at times, disappointed. (Which is not to say the "proofs" are not  valuable.)

I accept the RP, well, in principle. In practice--it is a very messy matter. One person's self-validated, RP compliant, God-honoring worship is another person's abomination. That is why the RP app comes in handy! Here is a screen shot (click to enlarge):

Okay, seriously, I thought about this yesterday when I saw, for the first time in about twenty years, an infant baptism. And it occurred to me that for the sacraments, the most important components of worship, we have no regulative principle. If anything we have an irregulative principle.

You cannot prove paedobatism or credobaptism from scripture. Both camps rely on arguments from silence. If someone tells you that baptismo always means immersion, ergo we should practice immersion, they are wrong. You can't even demonstrate that the phrase "came up out of the water" implies immersion--it can just as accurately be applied to someone who was standing ankle deep. Nor can you prove that baptism is the new circumcision. You can make plausibility arguments for both positions, but those arguments fall far short of any standard of proof.

Scripture commands us to baptize and to partake of communion. It is utterly silent on how or, for communion, how often. Now if you want to behold the mother-of-all regulative principles, in the form of a completely deterministic step-by-step do-or-literaly-die algorithm, you need look no further than the instructions for the priests in the Old Testament.

In fact, it is entirely tempting (and reasonable) to jump to the conclusion that the lack of detailed instruction in the New Testament, when it comes to the sacraments, is a feature, not a bug. Could it not be that God is telling us: You will baptize and you will serve communion, and as long as your church is faithful (to whatever level is possible for fallen humans) I will be present and I will dispense grace.

But man prefers to live under law rather than grace, and so we create our own conflicting rules for the sacraments because, well, it appears to us that God forgot to provide them! It is an irregulative principle. Every rule that  we add is of our own invention: no infants, yes infants, grape juice, no wine, hot water, cold water, sprinkling, immersion, natural water only, baptismals, years of preparation, minutes of preparation, complete testimony, canned yes/no questions, membership required, required for membership, unleavened bread, Wonder bread, gluten-free bread, once a week, once every two weeks, once a month...

Here is a multiple choice question:

Q: In what universe does it make sense to claim that baptism is a means of grace, and that whatever happens, happens of God, and it is all about God and not about the person being baptized and certainly not about the person performing the baptism, and yet some baptisms performed in God-honoring, gospel preaching, evangelical, inerrancy of scripture proclaiming churches are invalid because men performed them incorrectly, even though scripture is silent on the details?

A) Our universe
B) Some other universe in the multiverse
C) No universe

Submit your answer for grading.

Having said all that, I think it is perfectly reasonable (and consistent) for some churches to  be paedobaptisitic and some to be credobaptistic. But they should be honest that what they are doing is continuing a fine tradition in which some saints are comfortable. It is good to have both, because neither is prescribed, and each view should accept the baptisms performed in the other. Because, you know, unity is prescribed.