19 At last the command was given: Surrender the Basilica. My reply was, it is not lawful for me to surrender it, nor advantageous for you, O Emperor, to receive it. By no right can you violate the house of a private person, and do you think that the House of God may be taken away? It is asserted that everything is lawful for the Emperor, that all things are his. My answer is: Do not, O Emperor, lay on yourself the burden of such a thought as that you have any imperial power over those things which belong to God. Exalt not yourself, but if you desire to reign long, submit yourself to God. It is written: “The things which are God's to God, those which are Cæsar's to Cæsar.” The palaces belong to the Emperor, the churches to the Bishop. Authority is committed to you over public, not over sacred buildings. Again the Emperor was stated to have declared: I also ought to have one Basilica. My answer was: It is not lawful for you to have it. What have you to do with an adulteress? For she is an adulteress who is not joined to Christ in lawful wedlock.
Source: Letters (New Advent)