In his first homily as pontiff, he said he would use his papacy to “work unceasingly for the reconstruction of the full and visible unity of all Christians.”
He said his election had “confounded all my expectations”.
The pontiff, wearing the papal mitre on his head, led a solemn procession followed by 114 cardinals into the Sistine Chapel.
The mass, broadcast on television around the world, was otherwise a private ceremony for the cardinals that elected him.
The mass comes after one of the shortest conclaves in a century, which sent a signal that the church is intent on sticking to tradition by electing the ultra-conservative Benedict, formerly known as German cardinal Joseph Ratzinger.
The 78-year-old will be inaugurated as the 265th pope on Sunday.
He carried a pastoral staff and made the sign of the cross as he walked towards the mass.
The name Benedict comes from the Latin word for “blessing”.
The last Pope with the name reigned during World War I.
Ratzinger paid tribute to his predecessor, the late John Paul II, saying he had had left a “more courageous, freer and younger Church.”
“Confounding all my expectations, divine providence through the votes of the venerable father cardinals has called me to succeed this great pope.”
In his homily, delivered in Latin, he vowed to give bishops greater authority, divesting the Vatican of some of its power and allowing grassroots dioceses to set policy at a local level.
He said he is committed to open and sincere dialogue with followers of other religions.
“I address everyone with simplicity and affection to ensure that the Church wants to continue to weave an open and sincere dialogue with them in the search for the true good of man and society,” he said.
Ratzinger said he feels a sense of inadequacy at being elevated to the position, but can feel the presence of his predecessor guiding him.
“I seem to feel (John Paul II’s) strong hand holding mine, I seem to see his smiling eyes and hear him speaking to me at this moment, saying: ‘Don’t be afraid’,” he said.
“On one hand I have a sense of inadequacy and human turmoil at the responsibility entrusted to me yesterday … on the other hand, I feel living in me a deep gratitude to God who does not abandon his flock but guides them always.”
The pontiff also announced he plans to attend World Youth Day festivities in Cologne, Germany, in August.