This “of” is perhaps best understood to mean, “the armor that God himself wears, that is also available to you.” Isaiah 59:17 talks about the Lord (“Yahweh”) putting on righteousness like a breastplate and a helmet of salvation. Ephesians 6 mentions these same items.
The “armor” we are called to put on, then, is God’s own armor. We “put on” the attributes and characteristics of God. Because of his righteousness, we are called righteous. Because he is a God of peace, we can be at peace and make peace with others. Because of the faithfulness of Jesus, we can have faith.
And, what is more, God-as-Warrior is going on ahead of each of us, wearing that “armor,” fighting spiritual battles before us. As we follow him, we are given his same armor to wear.