Pope Benedict XVI has urged Christians to "find time and room for God in their fast-paced lives".
During Mass at St Peter's Basilica on Monday, he gave a homily around the theme of God's place in today's modern world.
"Do we have time and space for him? Do we not actually turn away God himself? We begin to do so when we have no time for him," he said.
"The faster we can move, the more efficient our time-saving appliances become, the less time we have. And God? The question of God never seems urgent. Our time is already completely full," he said.
Benedict, 85, said societies had reached the point where many people's thinking processes did not leave any room even for the existence of God.
"Even if he seems to knock at the door of our thinking, he has to be explained away. If thinking is to be taken seriously, it must be structured in such a way that the 'God hypothesis' becomes superfluous," he said.
"There is no room for him. Not even in our feelings and desires is there any room for him. We want ourselves. We want what we can seize hold of, we want happiness that is within our reach, we want our plans and purposes to succeed. We are so 'full' of ourselves that there is no room left for God."
Bells inside and outside the basilica chimed when the pope said "Glory to God in the Highest".
Earlier on Monday, he pope appeared at the window of the papal apartments and lit a peace candle, as a nativity scene was unveiled in St Peter's Square.
Reflecting on the gospel account of Jesus born in a stable because there was no room for Mary and Joseph in the inn, he said that when people find no room for God in their lives, they will soon find no room for others.
"Let us ask the Lord that we may become vigilant for his presence, that we may hear how softly yet insistently he knocks at the door of our being and willing.
"Let us ask that we may make room for him within ourselves, that we may recognise him also in those through whom he speaks to us: children, the suffering, the abandoned, those who are excluded and the poor of this world," he said.
Prayers for the Holy Land
Benedict also asked for prayers for the people who "live and suffer" in the Holy Land today.
The pope called for peace among Israelis and Palestinians and for the people of Syria, Lebanon and Iraq.
He prayed that "Christians in those lands where our faith was born may be able to continue living there, that Christians and Muslims may build up their countries side-by-side in God's peace."