Jumping in without doing any research at all, my opinion on this is that very little in Revelation is literal. So this mark probably isn't one thing or stamp, symbol, or one action.
People forget it's not the Devil's number, it's a human number.
Jesus talked about people getting married and doing business and getting on with everyday things, until suddenly the day was instantly upon them.
It seems to me to just be a general attitude of going about a life without any regards for spiritual things. A life where God is not a priority.