In how many other ways shall we yet further show that nothing which is peculiar to the shows has God's approval, or without that approval is becoming in God's servants? If we have succeeded in making it plain that they were instituted entirely for the devil's sake, and have been got up entirely with the devil's things (for all that is not God's, or is not pleasing in His eyes, belongs to His wicked rival), this simply means that in them you have that pomp of the devil which in the “seal” of our faith we abjure.
We should have no connection with the things which we abjure, whether in deed or word, whether by looking on them or looking forward to them; but do we not abjure and rescind that baptismal pledge, when we cease to bear its testimony? Does it then remain for us to apply to the heathen themselves. Let them tell us, then, whether it is right in Christians to frequent the show. Why, the rejection of these amusements is the chief sign to them that a man has adopted the Christian faith.
If any one, then, puts away the faith's distinctive badge, he is plainly guilty of denying it. What hope can you possibly retain in regard to a man who does that? When you go over to the enemy's camp, you throw down your arms, desert the standards and the oath of allegiance to your chief: you cast in your lot for life or death with your new friends.
Source: De Spectaculis (The Shows) (New Advent)