A.T. Robertson Commentary Ephesians 6:17

"And take the helmet of salvation, and the sword of the Spirit, which is the word of God:" — Ephesians 6:17 (ASV)

The helmet of salvation (την περικεφαλαιαν του σωτηριου). Late word (περι, κεφαλη, head, around the head), in Polybius, LXX, 1 Thessalonians 5:8; Ephesians 6:17 alone in N.T.

Which is the word of God (ο εστιν το ρημα του θεου). Explanatory relative (ο) referring to the sword (μαχαιραν). The sword given by the Spirit to be wielded as offensive weapon (the others defensive) by the Christian is the word of God. See Heb 4:12 where the word of God is called "sharper than any two-edged sword."