I have always taken Revelation 13:16-18 and Revelation 14:9-11 literally. And I still do. These are verses that describe the Mark of the Beast. My feeling is that if I can't trust the Mark of the Beast to be literal, then how do I know that the mark of Cain was literal? And if that mark was not literal, then maybe Cain and his mom and dad, Adam and Eve, were not literal either. And if I can't believe in a literal Adam and Eve, then what is the purpose of having the Bible at all?
No, I believe the Mark of the Beast will be a literal mark - maybe it will be a chip implanted under the skin, maybe it will be a literal 666. I don't know the details. But it will be a mark that a person will have to accept if they want to participate in the global economy (buy, sell, eat, etc.).
Having said that, I listened to a sermon today that challenged my long held belief in this literal mark, and I recommend you listen to it too. The thing that was said that I really want to draw your attention to is this:
"To practice worldly conformity is to receive the Mark of the Beast in all ages" - by Leon Morris (quoted by Stepan Holland, in Taking the Mark of the Beast).
I heartily recommend you listen to this thought provoking sermon
Taking the Mark of the Beast, by Stephan Holland