When Paul penned his famous passage on the armor of God, was he borrowing imagery from a Roman soldier's kit and gear? Hardly. That would have been a disastrous way to appeal to Christians oppressed by Rome.
The real inspiration for Paul's analogy is more scripturally consistent. In describing the believers' armor, the Hebrew of Hebrews was simply speaking in his mother tongue: the Old Testament.
God has made us a kingdom of priests. Christ, our great High Priest, makes it clear by example that our priestly duty is to be on the offensive against Satan's kingdom.
To fight, you want an armor suited for your needs. God's priests need a priestly armor.