"The unison voices of Christians would be more acceptable to God than any musical instrument." - Eusebius of Caesarea (263AD-339AD)
"Musical instruments were not used (in worship). the pipe, tabret, and harp here associate so intimately with the sensual heathen cults, as well as with the wild revelries and shameless performances of the degenerate theater and (Roman) circus..."-Augustine (354AD-430AD)In fact it wasn't until the mid 600'sAD that instruments, specifically an organ, was first introduced in worship by early Catholicism under Vitalian (657AD-672AD). So we can clearly see that it was Man who introduced instruments into the worship service. When man decides he knows better than God what God wants us to do to worship him the worship becomes adulterated and is invalid to God.
How important is it to only do those things which the Bible mentions we should do in worship? Well, consider Nadab and Abihu from the Old Testament.
"And Nadab and Abihu, the sons of Aaron, took each of them his censer, and put fire therein, and laid incense thereon, and offered strange fire before Jehovah, which he had not commanded them." Leviticus 10:1Both were sons of Aaron and both were priests who went to the temple to do their duty, but along the way they decided to do something different. Something God had not authorized. We don't know specifically what that was, only that it was offensive to God. The result was disastrous for the brothers.
"And there came forth fire from before Jehovah, and devoured them, and they died before Jehovah." Leviticus 10:2From this example we can see just how serious God expects us to be with our worship to him. But "what," you might ask, "does that have to do with using musical instruments in worship?" The answer is fairly straightforward... there is no mention of using instruments in worship in the New Testament or from early church history - only singing. When we introduce musical instruments as part of the worship then that part of our worship is offensive to God because it is not what he has commanded and when we insert our own human ideas into worship then our worship is in vain.