Former President Barack Obama on Thursday encouraged people not to “hide behind a wall,” making an oblique dig at President Trump during a talk in Berlin.

“In this new world we live in, we can’t isolate ourselves,” Obama said while on stage with German Chancellor Angela Merkel during a forum in Berlin, according to The Associated Press.

“We can’t hide behind a wall.”

Trump’s signature campaign promise was to build a wall on the U.S.-Mexico border to curb illegal immigration.

During the event, which marked the 500th anniversary of the Protestant Reformation, Obama told the audience that “we have to push back against those trends that would violate human rights or suppress democracy or restrict individual freedoms.”

{mosads}He also issued a warning about people who are always certain about their opinions, another apparent criticism of Trump.

“If I become so convinced that I’m always right,” Obama said, “the logical conclusion of that often ends up being great cruelty and great violence.”