Most Rev Justin Welby said his trip to the site in Poland made him ponder how to recognise and respond to evil.
At least one million people, mostly Jews, were murdered by Nazi officers at the Auschwitz camp between 1940 and 1945.
Gas chambers were used to kill the people who had been brought there on trains from across Europe.
"There are so many statistics about Auschwitz, but it defies description," said Archbishop Justin.
He was there as part of a group of Anglican clergy for "three days of prayer and theological and scriptural reflection".
He said on his Facebook page: "These atrocities were committed by ordinary people. When one of the priests leading our retreat was asked who was to blame, he said: "People did it to people."
"It was idolatrous and demonic. It was evil in the strict sense of human-created alternatives to the grace and providence of God.
"It reversed everything good with everything bad. During the retreat, the Revd Dr Sam Wells gave three extraordinary reflections on this question.
"I've come away with too much to write, and no words to write it. We must protest to the limit against evil: before it occurs, as it happens, and in its aftermath.
"But there is also a need for silent reflection, in which we honour the victims, mourn our capacity for evil, and learn to beware."
