They do not have children for fear of undermining their economic well-being or they try to justify it with other reasons, he said in his homily during morning Mass Dec. 19 at the Domus Sanctae Marthae.

The result is "countries empty of children. And this is not a blessing," he said. "Fruitfulness is always a blessing of God."

The day's readings spoke of two barren women who, with God's grace and intervention, each bore a son – Samson and John the Baptist.

Those readings and other passages show that "where there is God, there is fruitfulness," the Pope said.

Incidentally, he said, "some countries have chosen the path of barrenness and suffer from that terrible disease that is the demographic winter."

Fruitfulness can be "material or spiritual," he said, because it is about always giving life to others. People who do not marry can still be fruitful, he said. "But woe unto us if we are not fruitful with good works, if we do not bring fruitfulness to the people of God," he added.

"The devil wants sterility. He wants that each one of us live not for giving life – whether material or spiritual – to others," he said.

The devil wants people to live only for themselves and this "selfishness, pride and vanity is bloating the soul" by not living for others, he said. "The devil is the one who makes the weeds of selfishness grow and makes us not be fruitful."

With Christmas less than a week away, the Pope pointed to an empty crib, which was part of a Christmas creche.

That empty manger "can be a symbol of hope, because the child will come," he said, "or it can be a museum piece, empty" forever.

"Our heart is a crib. How is my heart? Is it empty, always empty" or is it "open to receive life continually and give life," he asked.

He asked people to pray before the empty Christmas crib during the last days of Advent and say, "Come Lord, fill the crib, fill my heart and encourage me to give life, to be fruitful."