Advocates for a capella (or non-instrumental) music in worship frequently turn to Ephesians 5:19 and Colossians 3:16. Neither of these prohibits instruments in so many words; rather, they describe the actions of worshipful, thankful hearts as they address their Father to offer Him praise. That said, these and every other passage in the New Testament that refer to music in the assembly mentions and emphasizes singing. Instruments are not mentioned at all.
This constitutes a pattern. Throughout God’s interactions with man, He has given instructions regarding His wishes for our worship, conduct, morality, and the like. The writer of Hebrews emphasizes this, quoting from Exodus 25:40 regarding the construction of the tabernacle: “for, ‘See,’ He says, ‘that you make all things according to the pattern which was shown you on the mountain’” (Hebrews 8:5). When specifics such as fabric, measurements, furniture, etc, were supplied, it was understood that all else would be excluded. God did not feel compelled to prohibit ever possible addition, nor did Moses supply one.
If a particular form of worship is emphasized and another is not, it makes us wonder what would motivate us to employ the one that is excluded. Yes, instruments were used in the Old Testament; the Psalms are replete with examples of it. But that being the case, why were they not used in the church? For centuries, instruments were excluded upon threat of excommunication. The very term a capella means “in the manner of the chapel” — that is to say, “like church music.”
True, many in the world today prefer to use instruments in their praise. But is that relevant? When did we use popularity among men as a gauge for acceptability before God? If God has given instruction and we are not content to remain within the bounds of that instruction, that may indicate an attitude problem that extends beyond our song worship.