Let us proceed and speak of the eating of meats. It is true that we are free to eat any manner of food, meats, fish, eggs or butter. This no one can deny. God has given us this liberty. That is true; nevertheless we must know how to use our liberty, and treat the weak brother differently from the stubborn. Observe, then, how you must use this liberty.
First of all, If you cannot give up meat without harm to yourself, or if you are sick, you may eat whatever you like, and if any one takes offense, let him be offended. And if the whole world took offense, yet you are not committing a sin, for God can excuse you in view of the liberty He has so graciously bestowed upon you, and of the necessities of your health, which would be endangered by your abstinence.
Secondly, If you should be pressed to eat fish instead of meat on Friday, and to eat fish and abstain from eggs and butter during Lent, etc., as the pope has done with his fools' laws, then you must in no wise allow yourself to be drawn away from the liberty in which God has placed you, but do just the contrary to spite him, and say: Because you forbid me to eat meat, and presume to turn my liberty into law, I will eat meat in spite of you. And thus you must do in all other things which are matters of liberty. (Fourth Sermon on the Wednesday after Invocavit)