666

The mark of the beast, 666, is not something that will take people by surprise, for it involves a certain attitude and belief system. 

As a physical identifier, I think it may be a thing of pride, placing it on the forehead. 

It is tempting to pursue the practice of Gematria in which numbers are represented by letters. The mark is presented as a physical identifier that involves the beast's name, and when that beast is manifested, his name will in some way add up to this 666. 

Irenaeus (second century) mentions that many names of contemporary persons and entities were being offered in his day as solutions to this number mystery. Yet he cautioned against the practice and believed that the name of the Antichrist was deliberately concealed because he did not exist in John's day. The name would be secret till the time of his future appearance in the world. Irenaeus expressly refutes the attempt of many to identify the name with any of the Roman emperors. He feels, however, that the gematria approach is John's intended meaning but warns the church against endless speculations. (EBC) 

The revelation given, concerning the mark, is that it is a point of no return: if you take the mark, your fate is sealed, this fact is a reason for the patience of the saints: see Revelation 14.9-13; 20.4-6. 

The mark is also a basis of discrimination for those special judgments that come on the earth: see Revelation 16.2 and the first bowl. It is mentioned as that which distinguishes the saints from the unsaved.