Is it a credit card? Your social security number? The bar code? RFID microchips? No, no, no and no. We must remember that while Revelation is partially about that which is to come, it was also very much about that which has already happened since it was a letter to seven communities 2,000 years ago, each with their own context.
It’s not unthinkable that John’s audience knew exactly what John meant when he referred to “the mark of the beast.” While scholars have differing views as to what “mark” John was referring to, many point to different ancient customs. For example, an Egyptian Pharaoh once required Jews living in Egypt to be branded with the symbol of a false god or be executed. Likewise, around John’s time it seems that some local officials made a formal requirement that if you hadn’t made the correct sacrifices, you were not able to trade in the market.
The mark of the beast is connected to the second beast which is connected to Rome, which was a nation with leaders that expected to be worshipped, which is connected to the idol worship of Satan and the false gods. If you took the mark of the second beast, you were consciously choosing to worship something or someone other than God. John’s mark was not some kind of science-fiction narrative like we often think of today. Rather, the mark was a sign that you had turned your back on God and worshipped idols. You had chosen Satan over Jesus.
So yes, the mark of the beast was likely something that John’s audience understood. To take on the mark was to be involved in the idol worship of the roman cult of Nero and his predecessors—false idols connected to the idol worship of Satan. But though this mark is in the past, anytime we are forced into idol worship today (and that includes the allegiance of nation and the worship of flag over the allegiance of Heaven and the worship of Jesus), we are taking on the mark of the beast. There is also much more symbolism in this passage that we don’t have time for in this post.