With about 8,000 pilgrims and visitors gathered in the Vatican audience hall, Pope Benedict spoke about the importance of the Week of Prayer for Christian Unity Jan. 18-25.

The lack of a united voice and united witness poses a huge obstacle to the new evangelization, "which would be more fruitful if all Christians proclaimed together the truth of the Gospel of Jesus Christ and gave a common response to the spiritual thirst of our age," the Pope said.

During his audience talk, Pope Benedict did not mention specific practical or moral issues dividing Christians today, but he has defined as obstacles to unity practices such as the ordination of women and different approaches to moral issues such as homosexuality.

The Second Vatican Council placed the search for Christian unity "at the center of the life and work of the church," the Pope said, and it did so because it was Christ's desire for his followers and because, practically speaking, it is essential for the full credibility of Christians.

"The lack of unity among Christians impedes a more effective proclamation of Christ because it puts our credibility in danger," the Pope said. "How can we give a convincing witness if we are divided?"

The key to Christian unity isn't simply to have members of different denominations be nice to one another and work together occasionally, he said.

"It requires that we reinforce our faith in God, the God of Jesus Christ, who spoke to us and became one of us. It requires entering into a new life in Christ, who is our true and definitive victory. It means opening ourselves to each other, welcoming all the elements of unity that God has preserved for us and gives us constantly. It means feeling the urgency of witnessing to the men and women of our time the living God who has made himself known in Christ," Pope Benedict said.