I have a special interest in the science of interpretation: Biblical or otherwise. Brethren correctly point out that we learn God’s will in the Bible by Commands, “Approved” examples, “Necessary” inferences. Let me say just a few words on these three before moving to your question. A command to preach the Gospel (Acts 20:18-20) or to be baptized (Acts 2:38), or to contribute (1 Cor. 16:1,2), is understood by most to be a command for us as well. Some would say that it’s really not a command to us, but these are authoritative to Christians today as “Approved” examples. An “Approved” example is where we learn from what they did, without a specific command. We learn to partake of the “Supper of the Lord” on the first day of the week by Paul’s example as recorded in Acts 19:6,7. If someone wants to say that this example is not binding, that person has to be asked what he is going to make his practice so as to create unity. An example is better than nothing! Actually, an example is better than anything (e.g. just look at the example of Christ’s life). And thirdly, seeking the will of God in a necessary inference is an acknowledgment that God reveals His will IMPLICITLY, not just explicitly. Except for the widow (1 Cor. 7:39), the Bible doesn’t say not to marry non-Christians, but it is implied that it isn’t the wise choice. Does God imply that the man or the woman is to be the main or primary breadwinner? It implies the man is to be the provider for his family and to work by the sweat of his brow while the woman is to be the nurturing mother. So what does the Bible imply on this matter and what must we infer? Here’s the thing, if God implies something, then we have a duty to properly infer from what He says. We must be careful about applying these principles of interpretation, that much is granted. When something is implied, then you are safe ground to speak such. If a Christian married a non-Christian, there is no command, example or implication to discipline the person, but it can still be said that the choice isn’t the wisest one.
But what about the expedient? I did a detailed study on this very subject on my blog. I invite you to check that out. But let me give you an answer here. An expedient calls for a degree of common sense or reason. If God says to do something, but He doesn’t say how, then how you decide to do it is the expedient. The expedient method you choose truly must be an expedient in that it must help to accomplish the will of God AND cannot contradict any other teaching. If God said for Abraham to go to Canaan, then he has no options on where he may go. But If God doesn’t specify exactly when he is to leave and arrive at the destination, then Abraham may choose to take his time in getting to his destination, UNLESS he learned through experience of watching others that God might view this as resistance to His will. It wouldn’t then be expedient to go slow. He might determine to move quickly is the will of God. Now when God says to go to Canaan, but doesn’t tell Abraham HOW to get to Canaan, then the chariot, carriage, camel, boat, or jet pack, or whatever means of transportation is available is an EXPEDIENT. Because the expedient cannot contradict or controvert some other teaching in the Bible. The expedient method the local congregation chooses must do no harm to other teachings of the Bible and should be what will promotes unity. The expedient method of travel does not constitute an addition to God’s word. God’s command, that doesn’t specify the transportation to be used, implies that Abraham has freedom to choose how to travel. Because Christians know they are to assemble, and there are examples of Christians in the Bible who met in various places (upper rooms, homes, public areas like Solomon’s portico), and not one specific meeting place as the Jews had at the temple, then Christians know they have some freedom on choosing WHERE to assemble. The choice of where to meet is an expedient. What about “spiritual songs”? God says we sing “psalms, hymns, and spiritual songs.” Does anyone want to fulfill this command without acknowledging that a collection of songs in the song book is an expedient? Having a song book for to fulfill the command to sing is like having a check book for fulfilling the command to give.
On the other hand, a song book is not a new kind of music or worship; but an instrument is a new kind of music. An expedient should do not harm to any other teaching in the Bible but musical instruments have done much harm. I know of no one who has ever been harmed by using a songbook. EVEN IF the music instrument could be shown to be a mere expedient, and not an addition to Scripture, it would still be shunned for the divisiveness it has brought. But instruments are clearly more than an expedient and do constitute a new form of music that is non-verbal, non-teaching, and non-edifying. God specified to “sing”, which is explicit and controlling on our worship activity. If God had said, “make music”, and had said nothing else on the matter, then there would be freedom to have playing and singing. But when God says “psalms, hymns, and spiritual songs”, it is the WORD OF GOD, which itself cannot be called legalistic, God specifies a particular kind of music which teaches and edifies. The instrument is not therefore appropriate because it cannot teach and edify in the Biblical sense. Denying the appropriateness of instruments in worship on this basis is different than denying their appropriateness on the reasoning that they encourage exhibitionism or vanity toward the player: singing could do this, but we don’t therefore not sing. Remember, Paul said, not I, to let “all things be done for edification.” The song book or a pitch pipe is simply a means of accomplishing what God has commanded: to sing.