WASHINGTON — Pope Francis used his first speech on US soil Wednesday to passionately call for action on climate change, plea for the US to protect the vulnerable, and offer US bishops support in their fight for religious freedom.

In an elaborate White House ceremony full of pomp and circumstance, President Barack Obama officially welcomed the pope to the United States, praising the pontiff’s humility in front of a crowd of more than 15,000 who had been gathered on the South Lawn since dawn.

The environment has become one of the pope’s primary concerns during his two and a half year papacy, culminating with the release of an encyclical, Laudato Si’, in June devoted to the topic.

For those who thought Francis may shy away from the climate change given the contentious nature of the debate here, his speech proved otherwise.

The pope said “climate change is a problem which can no longer be left to a future generation,” and cited Martin Luther King Jr. as an inspiration to act on the issue now.

“To use a telling phrase of the Reverend Martin Luther King, we can say that we have defaulted on a promissory note and now is the time to honor it,” he said.

Obama pledged his support for the pope’s efforts to rally world leaders in anticipation of a global summit on climate change in Paris in December.

“We support your call to all world leaders to support the communities most vulnerable to a changing climate and to come together to preserve our precious world for future generations,” he said.

President Obama | Pope Francis | Pope Francis in Spanish

If the pope’s words in support of action on climate change cheered progressive Catholics, his support of US bishops in their fight against the contraception mandate in Obamacare, as well as Christians who object to the legalization of same-sex marriage, gave comfort to the right.

Francis, who called himself “a brother of this country,” praised efforts to create a US society committed “to rejecting every form of unjust discrimination,” including religious discrimination.

“That freedom remains one of America’s most precious possessions,” he said. “And, as my brothers, the United States bishops, have reminded us, all are called to be vigilant, precisely as good citizens, to preserve and defend that freedom from everything that would threaten or compromise it.”

Obama also addressed religious freedom, but in reference to anti-Christian persecution in the Middle East and other areas of the world.

“Here in the United States, we cherish religious liberty,” he said. “Yet around the world at this very moment, children of God, including Christians, are targeted and even killed because of their faith.”

“People everywhere,” he said, “must be able to live out their faith free from fear and intimidation.”

Some in the audience teared up as Francis spoke about placing the vulnerable at the center of national concern: He called on Americans to support “to protect the vulnerable in our world and to stimulate integral and inclusive models of development.”

Absent from the pope’s speech was any explicit mention of abortion or same-sex marriage.

Obama cited his days as a community organizer in Chicago, a position funded by a grant from Catholic Charities, saying, “I’ve seen firsthand how, every day, Catholic communities, priests, nuns, and laity feed the hungry, heal the sick, shelter the homeless, educate our children, and fortify the faith that sustains so many.”

The president thanked Francis for his role in helping the United States and Cuba facilitate talks last year that led to the reestablishment of diplomatic nations between the two nations. The pope chose to precede his visit to the United States with a stop in Cuba, symbolically linking the two nations.

Francis gave previews of two other high-profile speeches.

He said that in Thursday’s speech to Congress, he will “offer words of encouragement to those called to guide the nation’s political future in fidelity to its founding principles.”

In Philadelphia, where he’ll travel Saturday, he’ll “celebrate and support the institutions of marriage and the family at this, a critical moment in the history of our civilization.”

The two leaders greeted each other warmly, smiling and shaking hands for several seconds after Obama’s remarks. This is their second meeting; Francis welcomed Obama to the Vatican last year.

Obama offered high praise to Francis, who has emerged as a sort of Catholic ally even as his relationship with American bishops remains fraught.

“In your humility, your embrace of simplicity, the gentleness of your words and the generosity of your spirit, we see a living example of Jesus’ teachings, a leader whose moral authority comes not just through words but also through deeds,” Obama said.

A choir from DC’s St. Augustine Church, an historically black Catholic Church, sang a hymn after the pope’s speech.

Vice President Joe Biden, the nation’s first Catholic vice president, sported his signature aviator sunglasses, frequently turning around during the ceremony to chat with cardinals seated behind him.

After the ceremony, First Lady Michelle Obama joined Francis and her husband on the balcony, waving to the crowd before entering the White House, where Obama and the pope were scheduled to hold a 45-minute meeting.

Invited guests began arriving at 5 a.m., subject to extensive security measures; each was given small Vatican and US flags. Military bands in red and blue uniforms welcomed the pontiff by playing the anthems of the Holy See and the United States, as the yellow and white flag of the Holy See waved next to the Stars and Stripes behind the two men.

Crowds erupted into applause on a cool, sunny morning when the president said that the nation’s 70 million Catholics were happy to host the pope.

Francis arrived in Washington yesterday after a brief greeting from Obama and Vice President Joe Biden at Joint Base Andrews in Maryland. He rode to the Apostolic Nunciature in a small Fiat, the same car that delivered him to the White House Wednesday.

From the White House, Francis will embark on a papal parade near the National Mall, where crowds have been lined up since before sunrise. He’ll use a popemobile crafted from a Jeep Wrangler.

After the parade, Francis will address US bishops at the Cathedral of Saint Matthew the Apostle before an afternoon Mass at the Catholic University of America to canonize Junipero Serra.