The following are all the passages in the New Testament that speak of music:
Matthew 26:30 (Mark 14:26)
"After singing a hymn, they went out to the Mount of Olives."
"But about midnight Paul and Silas were praying and singing hymns of praise to God, and the prisoners were listening to them;"
"and for the Gentiles to glorify God for His mercy; as it is written, "THEREFORE I WILL GIVE PRAISE TO YOU AMONG THE GENTILES, AND I WILL SING TO YOUR NAME."
1 Cor. 14:15
"What is the outcome then? I will pray with the spirit and I will pray with the mind also; I will sing with the spirit and I will sing with the mind also."
"speaking to one another in psalms and hymns and spiritual songs, singing and making melody with your heart to the Lord;"
"Let the word of Christ richly dwell within you, with all wisdom teaching and admonishing one another with psalms and hymns and spiritual songs, singing with thankfulness in your hearts to God."
"saying, "I WILL PROCLAIM YOUR NAME TO MY BRETHREN, IN THE MIDST OF THE CONGREGATION I WILL SING YOUR PRAISE."
"Is anyone among you suffering? Then he must pray. Is anyone cheerful? He is to sing praises."
In every one of the above passages the kind of music which God specified is vocal music. Whatever might be said in favor of instrumental music, no one doubts that we are worshipping God "in spirit and truth" (John 4:23,24) by simply "singing and making melody in our hearts" (Eph. 5:19) to the Lord.
We must always allow the Scriptures to speak to us, for they will give us a complete revelation of God's desires.
2 Tim. 3:16,17
16 All Scripture is inspired by God and profitable for teaching, for reproof, for correction, for training in righteousness;
17 so that the man of God may be adequate, equipped for every good work.
We are not to go beyond what is written in the Word of God.
1 Cor. 4:6
6 Now these things, brethren, I have figuratively applied to myself and Apollos for your sakes, so that in us you may learn not to exceed what is written, so that no one of you will become arrogant in behalf of one against the other.
The Bible commands us to "speak in song" (Eph. 5:19. The voice can obey this command, the instrument cannot.
The Bible commands us to "teach" and "admonish" in song (Col.3:16). The voice can obey this command, the instrument cannot.
The Bible commands us "sing with the spirit" (1 Cor. 14:15). The voice can obey this command, the instrument cannot.
The Bible commands that we "sing with the understanding" (1 Cor. 14:15). The voice can obey this command, the instrument cannot.
The Bible commands that we Praise God (Heb. 2:12, James 5:13). The voice can obey this command, the instrument cannot.
The Bible commands that we "make melody in our hearts" (Eph. 5:19). The voice can obey this command, the instrument cannot.
The human voice is capable of doing everything which God commands us to do as music in worship. The instrument cannot. We must follow the safe course. We can sing and fulfill all the requirements of Scripture to please God, or we can do something that does not fulfill the Scriptural requirements to please God, that is not mentioned in the Word of God, and which is an addition to the Word of God. Which one is the safest course? It's obvious, isn't it?
Therefore, Christians concerned with pleasing God in worship will refrain from using instruments.