Here’s how Jerome puts it:
From what we read of the Lord our Saviour throughout the Scriptures, it is manifestly clear that the whole armour of Christ is the Saviour himself. It is he whom we are asked to “put on.” It is one and the same thing to say “Put on the whole armour of God” and “Put on the Lord Jesus Christ.” Our belt is truth and our breastplate is righteousness. The Saviour is also called both “truth” and “righteousness.” So no one can doubt that he himself is that very belt and breastplate. On this principle he is also to be understood as the “preparation of the gospel of peace.” He himself is the “shield of faith” and the “helmet of salvation.” He is the “sword of the Spirit,” because he is the Word of God, living and efficacious, the utterance of which is stronger than any helmet and sharp on both sides.”
 Epistle to the Ephesians 6.11; cited in ACCS, p197.