U.S. Secretary of State Marco Rubio stressed on Sunday the challenge in negotiating with the Islamic Republic of Iran due to the “pure theology” that drives its decision-making.

“We’re dealing with radical Shia clerics and people who make geopolitical decisions on the basis of pure theology,” Rubio told reporters at the annual Munich Security Conference in Germany.

“It’s a complicated thing. No one’s ever been able to do a successful deal with Iran, but we’re going to try,” he went on to say.