Why do some people believe that churches shouldn't use musical instruments? Why is this wrong?
There's a view of church life that says we should only do what the Bible commands us to do in worship: the regulative principle. Another view says we are free to do whatever the Bible doesn't forbid us to do.
Now neither of those is completely clear, because virtually nobody only does what the Bible commands them to do. The Bible doesn't tell them what kind of shirt to wear. It doesn't tell them whether to stand at the front or the back when they preach. I mean, you could think of a hundred things we do in our worship that the Bible doesn't make clear that we should do.
And the people who say that we are free to do whatever the Bible doesn't prohibit have to come to terms with the reality that you have to work with principles. For example, preaching with a bathing suit on: the Bible doesn't forbid it, but if John Piper walked out with a bathing suit on then I couldn't help anybody! And so you've got principles of helpfulness that you use.
I think this fear of using instruments was based on the regulative principle understood a certain way that said that in the New Testament you don't find any instruments. ("Singing and making melody in your hearts to the Lord" [Ephesians 5:19] is there, but nobody is playing a harp or guitar or drum.) And I think that's a mistake to limit it that way.
Another reason why this decision was made is that accompanying music was considered an artificial way of arousing the affections. They argued that God should rouse your affections, and he should arouse your affections through truth, not through the artificiality of sounds that make us feel good.
Well, there is a real danger in sounds, but goodness gracious, if you're going to go that route then you've to do away with singing, because the human voice can make a beautiful sound that also is emotionally affecting.
God doesn't seem to be as afraid—whether with instruments in the Old Testament or voices in the New Testament—of possibly putting that stumbling block in our way. We just need to make sure that the accompaniment is suitable to the truth that we're saying and that we go through the sound and through the truth to God himself.