Pope Francis is becoming an aggressive public player in secular politics, from the environment to economic policy. That carries risks, not for Francis alone, but for the papacy and the institution the pope leads.

It is said widely that Francis will never allow himself to be co-opted into anyone else’s political agenda. The pope is famously his own man. But the pope has no control over whether he is co-opted into the political goals and strategies of others.

A TV commercial airing this week from NextGen Climate Action, funded by billionaire environmental activist Tom Steyer, unfurls frightening images of wildfires and floods and ends with Francis waving and smiling at us over the words, “With compassion and love—Pope Francis.” It’s propaganda, but legitimate propaganda by current standards.

The day before Pope Francis met with Mr. Obama, one of the president’s aides, Ben Rhodes, said: “How can we make use of the enormous platform that the pope’s visit provides to lift up the work we’re doing and demonstrate how it’s consistent with the direction that’s coming from the pope?” At the White House, Pope Francis praised Mr. Obama’s climate-change initiatives, and the president thanked the pope for supporting his policies on that and his opening to Cuba.