German national Oskar Groening, 94, stands accused of 300,000 counts of "accessory to murder" in the cases of deported Hungarian Jews sent to the gas chambers between May and July 1944.
He has acknowledged "moral guilt" but denied any legal culpability since the trial in the northern city of Lueneburg near Hamburg opened in April.
In a statement to the court read out by one of his attorneys Wednesday, Groening expressed his "humility and guilt before the survivors and victims' families".
He also stressed that he bore "shared guilt for the Holocaust, although my part was small". But he "expressly" stopped short of asking for "forgiveness for my guilt".
"In view of the scale of the crimes committed in Auschwitz and in other places, I do not believe I am entitled to make such a request," he said. "I can only ask my Lord God for forgiveness."
Wearing a purple sleeveless jumper and a grey dress shirt and trousers, Groening entered the courtroom with the help of two medics and using a walker.
Groening, who faces 15 years in jail, had indicated he wanted to address the court but has grown increasingly frail during the proceedings, which are tentatively scheduled to run until the end of July.
In previous testimony, he described in a clear, firm voice three occasions of "ramp duty" at the spot where deportees arrived by rail at the extermination and forced labour camp in Nazi-occupied Poland.
Groening served as a bookkeeper, sorting and counting the money taken from those killed or used as slave labour, collecting cash in different currencies from across Europe and shipping it back to his Nazi bosses in Berlin.
He said he had known at the time about the "mass murder" at Auschwitz but was unaware until his trial about the horror endured by the victims while they were still alive.
"For example I had no idea about the terrible conditions during the deportations - that shocked me," he said.
"It also became clear to me how much Auschwitz and the Holocaust influenced the lives of the witnesses I heard here. They have obviously suffered their whole lives from their experiences in Auschwitz and the loss of so many loved ones."
'All the horror'
As the final witness during the trial, Irene Weiss, 84, a Czech-born Auschwitz survivor from the United States, gave searing testimony against Groening.
Dressed in a black trouser suit and speaking in an unwavering voice, Weiss described her terrifying ordeal as a 13-year-old girl.
Showing two photographs of her family as they arrived at Auschwitz that were recovered 25 years after the Holocaust, Weiss said her mother, three younger siblings and older brother were all murdered soon after in the gas chambers.
Her father was forced to work in a Sonderkommando, removing corpses from the gas chambers and cremating them, until the SS shot him.
Weiss said she was unable to forgive Groening.
"He has said that he does not consider himself a perpetrator but merely a small cog in the machine," she said.
"But if he were sitting here today wearing his SS uniform, I would tremble and all the horror that I experienced as a 13-year-old would return to me.
"Any person who wore that uniform in that place represented terror and the depths to which humanity can sink, regardless of what function they performed."
Weiss accepted a hug from a fellow co-plaintiff when she finished her testimony and told reporters she was relieved. "The butterflies in my stomach are gone."
"My goal at this point in my life is to make sure this horrific event is not forgotten," she said.
Groening had previously been cleared by German authorities, but the legal basis for prosecuting ex-Nazis changed in 2011 with the trial of former death camp guard John Demjanjuk.
While previously courts had punished defendants for individual atrocities, Demjanjuk was convicted solely on the basis of having served at the Sobibor camp in occupied Poland.
Similarly, none of the witnesses testifying at Groening's trial have any personal recollection of him at the camp.
Some 1.1 million people, most of them European Jews, perished between 1940 and 1945 in the Auschwitz-Birkenau camp before it was liberated by Soviet forces.