The program was presented as euthanasia, but as the killing was mandatory, with more concern to economy than to dignity and relief of pain, the description as euthanasia is misleading.
Aktion T4 used the word Sonderbehandlung ("special treatment") as a euphemism for killing. The same word was used during the Holocaust. This complicates Holocaust deniers' claims that Sonderbehandlung meant something else.
Opposition by church figures
The Catholic church in particular likes to point out that it was opposed to the killings at the time (which however calls into question the "nobody knew anything" line of defense as to why nobody did anything about the Nazi crimes) and there were indeed some bishops, parish priests and others that spoke out against the killings. This opposition did in fact even result in the Nazis dialing down the killing for a time, which just goes to prove that not only was resistance possible, it would not have been futile.
The fact that the Catholic Church spoke out on behalf of disabled people but remained almost universally silent when it came to Jews should tell you all you need to know about contemporary Catholic anti-Jew sentiment.