VATICAN CITY — Pope Benedict XVI is marking the 50th anniversary of the Second Vatican Council, the church meetings he attended as a young priest that brought the Catholic Church into the modern world but whose meaning is still hotly debated.

Benedict was celebrating Mass on Thursday in St. Peter's Square and later will greet churchmen, including a dozen original Vatican II participants, re-enacting the great procession into St. Peter's that launched the council in 1962.

The anniversary comes as the church, beset by recent scandals, is fighting what it sees as a wave of secularism erasing the Christian heritage of the West and competition for souls from rival evangelical churches in Latin America and Africa.

The Pope has spent much of his pontificate seeking to correct what he considers the misinterpretation of Vatican II.