He made the call after arriving in Philadelphia, the City of Brotherly Love, for the final leg of his US visit.
After speeches to Congress and the United Nations earlier this week aimed at spurring world leaders toward bold action on immigration and the environment, he is expected to focus more heavily on ordinary Catholics during his two days in Philadelphia.
During a mass for about 1,600 people at the downtown Cathedral Basilica of Sts. Peter and Paul the Holy Father used his Homily to promote women in the Church.
He said the future of Catholicism in the United States requires a much more active role for lay faithful, especially women.
"It means valuing the immense contribution which women, lay and religious, have made and continue to make to the life of our communities," he said.
Francis has repeatedly said women should have a greater role in church leadership, though he has rejected the idea of ordaining women.
His praise of nuns marked his second such public expression of gratitude in the US after the Vatican under his leadership ended a crackdown on the main umbrella group of American sisters. Nuns in the cathedral appreciated the gesture.
"We have felt very strong support from him," said Sister Catherine Darcy of Merion, Pennsylvania, one of about 50 members of the Sisters of Mercy of the Americas at the Mass. "We feel he recognises the contribution that religious (women) make to the church throughout the world."
During the Mass the pope looked on as live performances were given by The Fray and Aretha Franklin.
On Sunday night Francis will celebrate Mass with an estimated one million people at Benjamin Franklin Parkway before flying back to Rome.