It is a reference to the Bible. In the Book of Revelation (13:11-18), it says:
For an explanation of the text, here are the notes of the Orthodox Study Bible:
"Then I saw another beast coming up out of the earth, and he had two horns like a lamb and spoke like a dragon":
"He performs great signs, so that he even makes fire come down from heaven on the earth in the sight of men":
"He causes all, both small and great, rich and poor, free and slave, to receive a mark on their right hand or on their foreheads":
"His number is 666":