Bishop of Münster, Clemens August Count von Galen, protested the T-4 killings in a sermon August 13, 1941. Thousands of copies were printed and circulated. Galen was not punished because Hitler did not want to clash openly with the Catholic Church.
Never under any circumstances may a human being kill an innocent person apart from war and legitimate self-defense. If you establish and apply the principle that you can kill 'unproductive' fellow human beings then woe betide us all when we become old and frail!... woe betide loyal soldiers who return to the homeland seriously disabled, as cripples, as invalids. If it is once accepted that people have the right to kill 'unproductive' fellow humans—and even if it only initially affects the poor defenseless mentally ill—then as a matter of principle murder is permitted for all unproductive people....
Then, it is only necessary for some secret edict to order that the method developed for the mentally ill should be extended to other 'unproductive' people, that it should be applied to those suffering from incurable lung disease, to the elderly who are frail or invalids, to the severely disabled soldiers. Then none of our lives will be safe any more. Some commission can put us on the list of the 'unproductive', who in their opinion have become worthless life. And no police force will protect us and no court will investigate our murder and give the murderer the punishment he deserves. Who will be able to trust his physician any more? He may report his patient as 'unproductive' and receive instructions to kill him. It is impossible to imagine the degree of moral depravity, of general mistrust that would then spread even through families in this dreadful German nation if God's holy commandment, 'Thou shalt not kill', which God proclaimed on Mount Sinai amidst thunder and lightning, which God our Creator inscribed in the conscience of mankind from the very beginning, is not only broken, but if this transgression is actually tolerated and permitted to go unpunished.
From Burleigh and Wipperman, The Racial State: Germany, 1933-1945 (New York, 1991), pp. 152-53.