The Pope's remarks according to the Boston Globe:

"When I came down, I greeted them, shook their hands and nothing more. This is good manners. It's called good manners and not getting mixed up in politics. If anyone thinks that greeting someone means getting involved in politics, they should see a psychiatrist," the pope said.
. . . .

The invitation to Sanders to address that session raised eyebrows when it was announced and touched off allegations that the senator lobbied for the invitation.

But the chancellor for the pontifical academy, Bishop Marcelo Sanchez Sorondo, said he invited Sanders because he was the only U.S. presidential candidate who showed deep interest in the teachings of Francis.

http://www.democraticunderground.com/10141417945

Both the Pope and Bernie are interested in drawing public attention to the lack of economic justice in the world today. The Pope is interested in this issue because most if not almost all of the message of Jesus if you actually read the four Gospels is about helping the poor, shunning excessive wealth and economic justice.

Bernie has been a vocal admirer of the Pope for a long time. There is nothing new about this. I don't believe that Hillary is the avid admirer of the Pope that Bernie is. She has not spoken of the Pope as often as Bernie has to my knowledge.

When it comes to economics, the Pope and Bernie agree on the central points. That is what is important here.