Okay, to your two questions:

1) Does a practice have to be explicitly mentioned in the New Testament, or can we assume OT practices continue unless explicitly abrogated?

Forget the term “regulative principle” for a second. Let me give you my first principle of interpretation when it comes to determining the polity and practice of God’s people throughout the Bible, including the church: first, ask WHO is authorized to do WHAT.

It’s my fundamental assumption that human beings—lumps of clay—are not free to do anything until God authorizes us, even eat an apple off a tree. Gratefully, he has authorized us to eat apples (Gen. 1:29), and so much more (Gen. 1:28; 2:18-25; 9:1-7; etc.).

Furthermore, I don’t assume he has authorized everyone in the same way, but he assigns different people with different offices: to Adam, one authorization; to Noah, another; to Abraham, still another; and so forth. And at any given moment, he provides different authorizations: one to a king, another to a priest, another to a prophet, another to the average citizen, another to the whole assembly of Israel when it gathers for the various festivals, and so on.

Jumping to the New Testament, then, it’s strange to me that people will point to what Paul does in Acts as an apostle and then immediately assume a normative lesson transmits to Christians or churches today. Well, hold on. Yes, Paul was given a job or an office: apostle. But why would we assume his job responsibilities and office authorities are ours? Maybe there is overlap. But the first question I as a church member or an elder need to ask is, What job assignment does God give me? What did he authorize me (as church member; as elder) to do? If I see the president of my company or the company attorney doing something, I would’t assume that I as a middle-manager am authorized to do the same things, would I? Of course not. So why would I assume I’m authorized to do everything that Paul does? 

A second and related hermeneutical principle is this: heed canonical horizons (where we are in the storyline) and covenantal administrations.

As the storyline of redemptive history progresses, God authorizes different groups of people differently with each covenantal administration: Israel one way; the new covenant community another.

One huge problem with prosperity gospel preaching, as you know, is that it pays scant attention to the progression of redemptive history and the Bible’s covenantal structure. So prosperity preachers often take promises given to people under the old covenant and then extend them to us under the new covenant directly, as if nothing has changed in redemptive history. Just think of the Prayer of Jabez!

But if I’m asking who is authorized to do what, I’m going to be particularly interested in the promises, authorizations,commissions, and commands given directly to us under the new covenant. Certainly the authorizations and commands given to Israel are relevant to us, too. But we need filter them through the fulfilling work of Christ in order to ascertain that relevance. For instance, the levitical food laws are in the Bible for me to read and learn from. But they don’t directly apply to me like they did to Israel. I have to understand those laws through their fulfillment in Christ. Jesus declared all foods clean, but these laws still teach me that God’s people should be holy and set apart (in whatever manner set-apartment is specified in each covenantal administration). These laws don’t teach me not to eat shellfish! (Thank goodness!) 

Put these two interpretive principles together, Andrew, and you’ll understand why I find the following statement by B. B. Warfield defending paedobaptism somewhat strange: “God established his church in the days of Abraham and put children into it. They must remain there until he puts them out. He has nowhere put them out. They are still then members of his church and as such entitled to its ordinances. Among these ordinances is baptism.”

This is like a high school student graduating, enrolling in a university, and then assuming everything in university will be the exact same except in those places where someone has told him that a certain high school practice stops. What?! You’re in college now. You’re under a different administration. Forget what you did in high school. Ask your university professors and administrators what you’re supposed to do. Admittedly, this illustration emphasises discontinuity, and if we wanted it to properly balance discontinuity and continuity we’d need to make God the Principal over both the high school and the college or something. But the point is: God is using a different institutional structure, and so I’m going to ask him, “Who have you authorized to do what on this campus?”

So, to your question: I don’t think we should immediately presume continuity, as Warfield does, but nor do I think we should presume discontinuity. Instead, I think we should let Jesus and the apostles be our guide to what’s continuous and what’s discontinuous. That’s my third hermeneutical principle.

In other words, I don’t assume any OT polity or practice binds me, or doesn’t bind me, until Jesus or the apostles tell me it binds me. I read the OT through their lenses. Do the 10 Commandments bind me? Well, in fact, 9 out of the 10 are repeated in the New Testament (guess which isn’t!). Does the law of the Garden not to eat a certain fruit, or the laws of the Nazarites, or the law of the Judges, or the law of the kings, or even the commands of the Psalms bind me? Only through Christ! If that were not true, you would never enter God’s presence because you don’t have clean hands and a pure heart! We take from the OT what Jesus and the apostles tell us to take from the OT.

Your example of Psalm 149 provides a perfect illustration: “Sing to the LORD a new song, his praise in the assembly of the godly! Let Israel be glad in his Maker; let the children of Zion rejoice in their King! 3 Let them praise his name with dancing, making melody to him with tambourine and lyre!”

Should we sing? Of course, we think. Paul tells us to sing, right? Should we do it the assembly of the godly? Well, which assembly do you mean? The assembly of Israel gathered for one of the three annual festivals, because that’s what the psalmist is talking about? No. In fact, verse 2, it appears he’s specifically talking to (ethnic) Israel and the (seminally conceived) children of Zion, and they are to rejoice in their (earthly) king. We don’t have one of those. At least not in the way that the Psalmist meant. Okay, uh, which of this applies to me and which doesn’t?

Well, I need to read these three verses through their fulfillment in Christ. I’m not going to assume that everything stays the same in how an Israelite would read this and how I should read it. Instead, I should recall that I’m under a different administration (not in high school anymore!), and so I’m interested in what Jesus and the apostles take from a text like this. Yes, Paul explicitly commands us to sing. Yes, Jesus explicitly establishes us in assemblies (Matt. 16 and 18), as do the apostles. Yes, Jesus declares himself my king. Yes, Paul even calls us “Israel” (through the promise; Gal. 3 and 6). But am I told to praise him with dancing in the assembly? Hmmm. Not explicitly. Maybe it’s implied by virtue of proximity to verses 1 and 2? What should I do with dancing?

Well, let’s keep reading the psalm. Verse 5: “Let them sing for joy on their beds.” Okay, I’m pretty sure I should NOT haul my bed into the church assembly, however fun that might be. Then verses 6 and 7: “Let the high praises of God be in their throats and two-edged swords in their hands to executive vengeance on the nations and punishments on the peoples.” Yeah, I’m definitely sure I’m NOT authorized to do that in a church assembly. (And no, I’m not going to allegorize the sword here to the sword of the word.)

Bottom line: it’s pretty clear that Psalm 149 does not mean to give us a set of instructions for what to DO in church services today. Nor does any psalm. That’s not what they were written down for. Instead, let’s take from the psalms what Jesus and the apostles tell us to take, including singing psalms, like Psalm 149 (Col. 3; Eph. 5).

Goodness, that took way too long. I’m sorry. You’re patient. Your second question: 

2) Where is the line between an “element” and a “form”? Or: when does the form of doing something actually become a new “thing” or element?

That’s my interpretation of your question. Am I understating you correctly? This brings me to a fourth hermeneutical principle: if we start by asking the Bible WHO is authorized to do WHAT, we must then employ wisdom for determining HOW to fulfill an authorization.

If the Bible specifies the WHO and the WHAT, it doesn’t typically specify the HOW, at least not in concrete situations.
• It tells pastors to preach the Word, it doesn’t tell us exactly how. Using illustrations? Always verse by verse? The New Testament doesn’t say. So we use wisdom.
• It tells congregations to sing, it doesn’t tell us exactly how. This style? That style? With instruments or amplification or without? Again, the New Testament doesn’ say. Let’s use wisdom.
• It tells elder to oversee, it doesn’t tell them how. Visiting members in their home? Making appointments in their offices? Meeting weekly as elders on Thursday nights? Let’s use wisdom.
An element is like a piece of furniture. A form is like the style of furniture. So I’m turning to Scripture to ask, “Which pieces of furniture need to be in this room?” But I’m going to use wisdom in determining the style of furniture: “Should we upholster that couch in leather or fabric?”   

A football (soccer) analogy: the rules tell the forward (WHO) that he needs to get the ball down the field and kick it into the goal (WHAT). But HOW he does that (dribble left? dribble right? pass?) will depend on the idiosyncratic circumstances of the moment (the strengths of his opponents; the strength and health of his teammates, etc.). 

Elements should be in all churches everywhere (because they are in the Bible); forms depend on context and wisdom (because the Bible just doesn’t say). And to be clear, Andrew, everyone has some criteria for what they would or wouldn’t include in a church service. I’m just trying to make make those criteria explicit and consistent and biblical.

What’s the rule for what constitutes an acceptable form versus an unacceptable form? Well, the form of something must maintain the basic integrity of that which makes an element an element. For instance, can we call performance dancing a form of the element of teaching? Well, in the Bible, teaching always involves words, so, no, it does not seem to fulfill what makes teaching teaching. Is the use of instruments and amplification an acceptable form of singing? Well, insofar as it actually helps the people to fulfill the command to sing, absolutely. At some point, however, the instruments can begin to overwhelm the signing (whether we’re talking about loud electric guitars or pipe organs). When that happens, your form actually begins to violate the integrity of the element and we would at least call it unwise.

Instead, I’m going to look for forms that BEST implement the element, which is why I keep calling for wisdom. What forms of preaching BEST implement Paul’s command to “preach the Word”? Well, I think I can do that through topical preaching (a form). And so I might do that sometimes. But on the whole, I can best do that (I believe) through expositional preaching (another form). Therefore I will make that the regular diet of the church.  What about announcements? I would place those in the “form” or “how” category because they are an implicit and necessary component of gathering people together in an orderly fashion. And the gathering, of course, is an element.

Final thought: what then do we do with dancing? Clearly (I propose) it’s nowhere presented as an element by Jesus or the apostles for the gathering of the church. For that reason, I just would never support, say, performance dancing on stage in a church service. At that point, you have a new “thing.” A new element.

But what about swaying or even jumping up and down in the church pew while singing? Might we just call that a form of the bodily expression of singing and praise? Yeah, maybe. I don’t think I have a problem with that, at least I don’t have a problem formally. It might be unwise in some contexts. But in others I trust it’s fine.

So I was an interim pastor in the Caribbean once upon a time. There I stood in the front row during the music, standing perfectly still, like a stiff White American, hands at side, while everyone around me swayed. It was great! I was happy for them to be them, and they were happy for me to be me. And together we praised God.

But now suppose someone (let’s call him Andrew) stepped out of the pew and began to dance in a way that drew particular attention to himself. At some point, that dancing would become a different thing. A new element. People could say, “Do you remember when Andrew was dancing like that? Wow! He had some moves!” Where precisely is the line between the first and the second? Between moving your body in a culturally-typical manner of singing and actually dancing as a thing or an element? In my mind, that’s something of a judgment call, and I don’t feel the need to nail down precisely where that line is for every single church practice. I would just say,
• “let all things be done for the building up” (1 Cor. 14:26);
• make sure “the spirit of the prophets are subject to the prophets” (v. 32);
• and “all things should be done decently and in order” (v. 40).
In other words, feel free to physically express your praise in a way that’s normal in your culture, but make sure whatever you do builds others up, is under control, and remains orderly.

Well, I pray all this is useful. I look forward to hearing what you think. Thanks for reading (if you’re still here).

Blessings,
Jonathan