In Ephesians 5:19 we read, "Speaking to yourselves in psalms, hymns, and spiritual songs, singing and making melody in your heart to the Lord." Also we read in Colossians 3:16, "Let the word of Christ dwell in you richly in all wisdom; teaching, and admonishing one another in psalms, hymns and spiritual songs, singing with grace in your hearts to the Lord." Here we see in our singing, we are to speak, teach, and admonish one another. This is what God expects us to do when we sing.
But can a mechanical instrument of music speak, teach, or admonish as God wishes us to do when we sing? No. A mechanical instrument of music cannot accomplish anything God has commanded when we sing to Him in worship. Here we see that when we sing, we are "to sing with grace in your hearts" and "make melody in your heart." The melody God wants is to come from the human heart and not from lifeless mechanical musical instruments.
Some people say, "I like instrumental music in worship". It becomes: "I like it, I want it, and I am going to have it." Their feelings take precedence over what God has authorized. They are trusting in themselves. In Proverbs 28:26 we read, "He who trusts in his own heart is a fool."
We read in 2 John 9, "Whosoever transgresses and does not abide in the doctrine of Christ does not have God." We can not use instrumental music by the doctrine or authority of Christ, because Christ has nowhere authorized it. If we add mechanical instruments of music to the worship of God then we have added to God’s word. In the closing words of the New Testament we are warned not to add to His word. In Revelation 22:18, "If anyone adds to these things, God will add to him the plagues that are written in this book."
If we follow only what God has authorized in our singing in worship, then the whole congregation will sing without mechanical instruments of music. Only then can we be pleasing to God so we can go to Heaven.