The Pope has stated that the right to religious liberty is "one of America's most precious possessions".
He told nearly 15,000 guests assembled on the lawn of the White House, Washington that all are called to be vigilant and good citizens "to preserve and defend that freedom" from anything that would threaten or compromise it.
Prompting cheers from the crowd, he said the United States was a country built on immigration, and that he too had been an immigrant. He also spoke about the importance of facing up to climate change and added that during his six-day visit he will be talking about marriage and family life.
He commended American Catholics for supporting attempts to build a society that is "truly tolerant and inclusive", including safeguarding the rights of individuals and communities and rejecting unjust discrimination.
Prior to the event, the Vatican was among those understood to have objected at the guest list which included gay and transgender activists such as the retired episcopal bishop Gene Robinson.
President Barack Obama agreed that people are only free if they can practise their faith freely. Obama said: "We stand with you in defence of religious freedom."
Pope Francis is expected to say tomorrow that he does not want to abolish the market of capitalism, but to purify it.
The Pope, the first ever to address Congress, will attempt to clarify his views on economics and politics.
Pope Francis is viewed with growing concern by the American right, because of his apparently socialist take on capitalism and the markets. He has repeatedly said that money is the "devil's dung" and spoken up for the poor, marginalised and deprived while attacking capitalist economics.
However on arriving in Washington on Tuesday, Pope Francis insisted he was not left wing but merely following the social teaching of the Catholic Church.
The likely content of the widely-anticipated speech was disclosed by Catholic News Agency who spoke to an unnamed source who saw early drafts of the speech.
"Pope Francis will likely use the speech to Congress to clarify his position on economics and politics, thus indirectly responding to the misinterpretation of his words," the source said. "Pope Francis will clarify that he never said he wants to abolish the market, but that he said the market needs a purification."