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While the entire six-day U.S. trip is notable in and of itself, his address to a joint meeting of Congress is particularly significant because none of the three previous popes who visited the United States did so.
Francis began this morning by reminding lawmakers of their "responsibility as members of Congress is to enable this country, by your legislative activity, to grow as a nation."
He added: "Legislative activity is always based on care for the people. To this you have been invited, called and convened by those who elected you."
The pope, speaking in English, addressed a wide range of subjects, including Cuba, the U.S. civil rights movement and war.
"Here we have to ask ourselves: Why are deadly weapons being sold to those who plan to inflict untold suffering on individuals and society? Sadly, the answer, as we all know, is simply for money: money that is drenched in blood, often innocent blood," he said.
On the subjects of justice and peace, the pope visibly moved Rep. John Lewis, D-Ga., who, as a college-aged civil rights leader, helped lead an orderly march across the Edmund Pettus Bridge in Alabama, as he read this line: “Here too I think of the march which Martin Luther King led from Selma to Montgomery fifty years ago as part of the campaign to fulfill his 'dream' of full civil and political rights for African-Americans.”
People sitting next to Lewis rubbed his shoulders proudly.
In the remark most directly referring to Cuba, in whose re-established diplomatic relations with the United States Francis played an instrumental role, he said, “When countries which have been at odds resume the path of dialogue – a dialogue which may have been interrupted for the most legitimate of reasons – new opportunities open up for all.”
Whatever emphasis Francis had difficulty making via his phonetic pronunciation of the words on the page, he made up with his physical delivery, tapping his hand gently, but forcefully, on the podium as he said “new opportunities for all.” It elicited light applause.
In s comment on arms control that made the room feel more like a State of the Union, Francis said, “In the face of this shameful and culpable silence, it is our duty to confront the problem and to stop the arms trade.”
About half of the room stood up; the other half remained seated.
House Speaker John Boehner is playing host to the pope while he is on Capitol Hill but, as he has throughout the trip, Vice President Joe Biden is present. Biden and Boehner of Ohio are the two highest-ranking U.S. elected officials who are Catholic.
The House Chamber was packed, and was expected to include seven of the 2016 presidential candidates: Sens. Ted Cruz, Rand Paul, Marco Rubio, Lindsey Graham and Bernie Sanders, in addition to retired neurosurgeon Ben Carson and N.J. Gov. Chris Christie.
When Pope Francis arrived, he was escorted into an office where Boehner was waiting to meet with him. They sat and posed for photos, having a quick exchange over Boehner's green tie before all reporters were ushered out of the room.
ABC News' Ali Weinberg contributed to this story.