The "mark of the beast" is a number of mankind. We are the "beasts" and the mark is how we choose to live and the the people we choose to lead us.
If we choose to live on a "higher/moral/ethical/spiritual level" then we won't have the "mark of the beast" written upon us.
If we choose to live as "beasts" then the mark will be all over us - on our foreheads (the way we think) and on our hands (the way we act).
In the book of Revelation, John was referring to the persecution of people (mostly Christians) by the Roman Empire - specifically the Emperor Nero. Caesar Nero translates to the number 666 (Depending on the source, it also could translate to 606 in the Bible...so all the "wannabe Satanists" that use 666 as some sort of creed, could be completely mistaken) ...according to ancient numerology and various Bible translations.
Why did John use so many "criptic" messages in Revelation?
Because at the time, there was no such thing as "Free Speech". Had John written the Book of Revelation with simple and direct language, he would have been killed and the book would have been destroyed before anyone had a chance to read it.
The book of Revelation is a spiritual masterpiece but unfortunately, it has been taken out of context by many.
People of faith should focus on the overall message of Revelation - which essentially tells people of faith to stay strong and to not lose their faith. Because in the end, if you stay strong and keep your faith, you will have God's mark upon you. (Your faith and the way you think, act, etc.) - on your forehead.
Those that wander and lose their faith will have the "mark of the beast" upon their foreheads (The way they think) and on their hands (The way they act).