In Christ you also, when you heard the word of truth, the gospel of your salvation, and believed in him, were sealed with the promised Holy Spirit. (Ephesians 1:13)

The word translated ‘sealed’ is the Greek verb σφραγίζω sphragizó describing the act of fixing a seal or stamp to something attesting to ownership.  This conveys the full authority, backing and security of the owner and at one time might also involve the use of tattoos; hence the derivation of this prophetic commandment to the world that John witnessed in his revelation: