In Ephesians 5:19, Christians are told to speak "to one another in psalms and hymns and spiritual songs, singing and making melody in your heart to the Lord" (NKJV). Does the phrase "making melody" (5567, psallo ψάλλω) include singing with musical accompaniment? Or does the fact that Christians are to make melody "in your hearts" mean the heart is the instrument in this verse? Thanks!
The word ψάλλω (psallo) occurs only five times in the NT (Rom 15:9, 1 Cor 14:15, Eph 5:19, James 5:13) and always means "sing" as with the human voice.
However, it is well known that the verb ψάλλω originally meant to "pluck" or "play an instrument" (see BDAG and Thayer). Thus, BDAG defines ψάλλω in the NT as:
"sing songs of praise, with or without instrumental accompaniment".
Thus, the occurrences of ψάλλω in the NT neither necessarily include nor exclude the use of musical instruments.
I note that in Rev 5:8, 14:2, 18:22, the righteous sing praises accompanied by "harps", the Greek word for which is κιθάρα (kithara) from which comes our English word, "guitar".
The phrase in Eph 5:19 about "in your hearts" begins with the dative article, τῇ (tē); thus the phrase could be rendered, "from your hearts" (eg, NIV, HCSB), or, "in your hearts" (eg, NLT, BSB), or, "with your hearts" (CSB, CEV, NASB), etc. [My personal preference would be "with" or "from" without entirely excluding "in" either.]