The fifth of Paul's Epistles, and the tenth book in the New Testament, is Ephesians. It was written some time around 61 AD whilst Paul was a prisoner in Rome, awaiting his trial by the Emperor Nero; a persecutor of Christians.
Paul's letter to the Church at Ephesus (in modern day Turkey) is not an admonishment, like some of his other letters, but rather an encouragement. Paul describes the nature of the Church as a living thing; the body of Christ. He then tells the Ephesians to act like Christ's body on earth.
"Finally, my brethren, be strong in the Lord, and in the power of his might. Put on the whole armour of God, that ye may be able to stand against the wiles of the devil. For we wrestle not against flesh and blood, but against principalities, against powers, against the rulers of the darkness of this world, against spiritual wickedness in high places. Wherefore take unto you the whole armour of God, that ye may be able to withstand in the evil day, and having done all, to stand. Stand therefore, having your loins girt about with truth, and having on the breastplate of righteousness; and your feet shod with the preparation of the gospel of peace; above all, taking the shield of faith, wherewith ye shall be able to quench all the fiery darts of the wicked. And take the helmet of salvation, and the sword of the Spirit, which is the word of God"