The 77-year-old pontiff gave an interview to Il Messaggero, Rome's local newspaper, to mark the feast of Saints Peter and Paul, a Roman holiday.
He was asked about a blog post in the Economist magazine that said he sounded like a Leninist when he criticized capitalism and called for radical economic reform.
"I can only say that the communists have stolen our flag. The flag of the poor is Christian. Poverty is at the center of the Gospel," he said, citing Biblical passages about the need to help the poor, the sick and the needy.
"Communists say that all this is communism. Sure, twenty centuries later. So when they speak, one can say to them: 'but then you are Christian'," he said, laughing.