VATICAN CITY — Christians must recognize that they do not lead or guide the church, but that the Holy Spirit does and the Holy Spirit can be unpredictable, Pope Francis said.

"If, for example, an expedition of Martians arrived tomorrow," and one said he wanted to be baptized, "What would happen?" the pope asked May 12 during his early morning Mass at the Domus Sanctae Marthae.

Explaining that he really was talking about Martians, something unimaginable, he said he meant beings that are "green, with long noses and big ears, like in children's drawings."

According to Vatican Radio, Pope Francis said that if the Holy Spirit prompted the most unusual being to seek baptism, who would we be to hinder that person?

The pope focused his homily on the day's first reading, Acts 11:1-18, which tells of the Apostles' discussion -- and consternation -- over the Holy Spirit descending on a group of Gentiles at a time when the rest of the community of believers came from the Jewish tradition.

From the very beginnings of Christianity, the pope said, church leaders and members have been tempted at times to block the Holy Spirit's path or try to control it.

"When the Lord shows us the way, who are we to say, 'No, Lord, it is not prudent! No, let's do it this way,'" he said. "Who are we to close doors?"

Many parishes, Pope Francis said, have ushers to open the church doors and welcome people in, "but there has never been a ministry for those who close the doors. Never."

"The Holy Spirit is the living presence of God in the church," he said. Jesus sent the Spirit after his ascent into heaven to guide the church and lead it forward into uncharted territory.

The Spirit "makes unthinkable, unimaginable choices," the pope said. "And we Christians must ask the Lord for the grace of docility to the Holy Spirit" in order to follow the Spirit's lead.