There is a teaching that says "adding or subtracting" from God's word is a sin. My question is how can you add to the word?
My congregation teaches that if you add musical instruments to worship, you are adding to the word because the word states "to sing and make music in your hearts".
The word states that "Now on the first day of the week when the disciples came together to break bread." If we add another assembly on the first day of the week, are we adding to the word? Also is "assembling" singular and "assemblies" plural?
How are we to hold commands over different situations?
Your question is worded as if the teaching is something men created, but consider:
"You shall not add to the word which I am commanding you, nor take away from it, that you may keep the commandments of the LORD your God which I command you" (Deuteronomy 4:2).
"Therefore you shall be careful to do as the LORD your God has commanded you; you shall not turn aside to the right hand or to the left" (Deuteronomy 5:32).
"Whatever I command you, you shall be careful to do; you shall not add to nor take away from it" (Deuteronomy 12:32).
"Only be strong and very courageous; be careful to do according to all the law which Moses My servant commanded you; do not turn from it to the right or to the left, so that you may have success wherever you go" (Johsua 1:7).
"Be very firm, then, to keep and do all that is written in the book of the law of Moses, so that you may not turn aside from it to the right hand or to the left" (Joshua 23:6).
"Do not add to His words Or He will reprove you, and you will be proved a liar" (Proverbs 30:6).
"But even if we, or an angel from heaven, should preach to you a gospel contrary to what we have preached to you, he is to be accursed! As we have said before, so I say again now, if any man is preaching to you a gospel contrary to what you received, he is to be accursed!" (Galatians 1:8-9).
"I testify to everyone who hears the words of the prophecy of this book: if anyone adds to them, God will add to him the plagues which are written in this book; and if anyone takes away from the words of the book of this prophecy, God will take away his part from the tree of life and from the holy city, which are written in this book" (Revelation 22:18-19).
God does not want His commands changed by men. When people say acknowledge that God commanded something back then, "but times have changed!" they are saying trying to justify changing God's commands.
In regards to instrumental music in worship, please show the command that allows them in Christian worship. I know of commands that state we are to sing (Ephesians 5:19; Colossians 3:16), but the lack of authority for using them in New Testament worship is significant, especially when you consider that God was clear in the Old Testament that instruments were to be used in the worship of the Israelites. I choose to respect what God said and not make assumptions (additions) where God did not speak.
There is a description of the meeting for worship: "And they continued steadfastly in the apostles' doctrine and fellowship, in the breaking of bread, and in prayers" (Acts 2:42). This breaking of bread refers to the Lord's Supper. The Lord's Supper is to be partaken by the entire congregation together (I Corinthians 10:16-17; 11:20-21, 33). Remember that 3,000 were added to the church on the first day. There would not be many homes that could accommodate this congregation. But we see in the passage that these people did gather at the temple where room could be found for such a large assembly. Acts 2:46 tells us they gathered at the temple on other days as well. Therefore, there is authority for gatherings other than just once on Sundays for the partaking of the Lord's Supper.
In the command, "Let us hold fast the confession of our hope without wavering, for He who promised is faithful; and let us consider how to stimulate one another to love and good deeds, not forsaking our own assembling together, as is the habit of some, but encouraging one another; and all the more as you see the day drawing near" (Hebrews 10:23-25). If "assembling" was singular, it would refer to only one gathering -- not one gathering per week. The implication in Hebrews 10:25 is that Christians are not to make a habit of skipping services because we need encouragement. How frequently those services are held is not mentioned in this passage. There is the main service held once a week (Acts 20:7; I Corinthians 16:2), but a local congregation can and does meet at other times as well.
Whenever a command is given, it creates a boundary. The command excludes things not commanded while at the same time including things that fall within the command. See Must It Be Spelled Out? for details and examples. Adding is when you include something a command did not authorize. Subtracting is when you don't do something that was commanded.
Thank you so very much for your answer.