Isaiah 59.17 - He put on righteousness as his breastplate,and the helmet of salvation on his head; he put on the garments of vengeance and wrapped himself in zeal as in a cloak.

Paul is pulling from Isaiah's description of the Messiah. But now pictures us, the church, as wearing the very armor of God.

Ephesians 6:10, Ephesians 6:11-13, Ephesians 6:14 and Ephesians 6:15-17