The theme of the New Year's Mass led by Pope Francis at St. Peter's Basilica was "No Longer Slaves, but Brothers and Sisters."

Francis used the World Day of Peace service as an opportunity to condemn slavery and human trafficking across the globe.

"All of us are called [by God] to be free, all are called to be sons and daughters, and each, according to his or her own responsibilities, is called to combat modern forms of enslavement," he insisted.

"From every people, culture and religion, let us join our forces."

The call for unity against injustice echoes remarks Francis made last month in regards to sweatshops. He urged consumers to think twice before purchasing low-cost goods and material that may have been manufactured under oppressive means.

Francis also came to the defense of underprivileged populations and exploited workers during a New Year's Eve Vespers service. He criticised Rome politicians accused of stealing funds meant to help migrant workers, and called for a "spiritual and moral renewal" in the ancient city.

"We need a great daily attitude of Christian liberation," the pope said, "to defend the poor rather than defending ourselves from the poor, and to defend the weak rather than defending ourselves from the weak."

Many of those attending the Pope's first address of 2015 had participated in a peace march to the Vatican. Francis said that peace can be more than just a goal.

"Peace is always possible but we have to seek it," he said. "Let us pray for peace."