Irene Gut Opdyke said this quote

We did not speak of what we had seen. At the time, to speak of it seemed worse than sacrilege: We had witnessed a thing so terrible that it acquired a dreadful holiness. It was a miracle of evil. It was not possible to say with words what we had witnessed, and so we kept it safely guarded until the time we could bring it out, and show it to others, and say, 'Behold. This is the worst thing man can do'.

~ Irene Gut Opdyke