God the Almighty almost always chooses the least mighty people and the most desolate places to reveal the power of his love, Pope Francis said.

Celebrating Mass Dec. 5 in Athens' Megaron concert hall, the pope touched on a theme he had explored in depth with Catholic leaders the day before: the blessing and spiritual advantage of being a small community without power and without pretenses.

Catholics make up less than 2% of the population of Greece; more than 90% of the country's residents belong to the Orthodox Church.

Noting how the day's Gospel says the word of God came to John the Baptist "in the desert," Pope Francis said, "There is no place that God will not visit."

"Today we rejoice to see him choose the desert, to see him reach out with love to our littleness and to refresh our arid spirits," he said. "Dear friends, do not fear littleness, since it is not about being small and few in number, but about being open to God and to others."

The late-afternoon Mass was the pope's last public event in Greece. After Mass he was to host a private visit by Orthodox Archbishop Ieronymos II, head of the Orthodox Church of Greece, and the next morning he was scheduled to visit a Catholic school before returning to Rome, concluding a five-day trip that began in Cyprus.

Celebrating Mass in the Greek capital after having flown to and from the outlying island of Lesbos for a meeting with migrants, Pope Francis' homily focused both on recognizing God at work where he is least expected and on the Advent challenge of conversion.

St. Luke's description of the call of St. John the Baptist lists the civil and religious leaders in office at the time. "We might have expected God’s word to be spoken to one of the distinguished personages" mentioned in the reading, the pope said. "Instead, a subtle irony emerges between the lines of the Gospel: from the upper echelons of the powerful, suddenly we shift to the desert, to an unknown, solitary man."

"God surprises us," the pope told the 2,000 people at Mass. "His ways surprise us, for they differ from our human expectations; they do not reflect the power and grandeur that we associate with him. Indeed, the Lord likes best what is small and lowly."

The Gospel teaches that "being powerful, well-educated or famous is no guarantee of pleasing God, for those things could actually lead to pride and to rejecting him. Instead, we need to be interiorly poor, even as the desert is poor."

The day's reading also called for conversion, something that sounds difficult, he said, because too many people think of it as a rallying of personal strength in a struggle for perfection.

But the Greek word for conversion -- "metanoia" -- means "'to think beyond,' to go beyond our usual ways of thinking, beyond our habitual worldview -- all those ways of thinking that reduce everything to ourselves, to our belief in our own self-sufficiency," he said.

"To be converted, then, means not listening to the things that stifle hope, to those who keep telling us that nothing ever changes in life," the pope said. "It means refusing to believe that we are destined to sink into the mire of mediocrity."

"Everything changes when we give first place to the Lord. That is what conversion is," Pope Francis insisted. "As far as Christ is concerned, we need only open the door and let him enter in and work his wonders."