666 is most commonly known as "The Number of the Beast". It is most commonly associated with The First Beast of The Apocalypse, more commonly known as The Antichrist.

In most manuscripts of the New Testament and in English translations of the Bible Chapter 13 Verse 18, it describes following the emergence of the second beast of the apocalypse (The False Prophet) from the earth, it will demand that every living human being worships the Antichrist or be put to death. Those who renounce their faith in God would receive the Mark of The Beast on either their right hand or forehead. According to the Bible, anyone who does not have the mark and the ability to count it will be unable to participate in commerce or democracy.

There have been many interpretations as to what The Number of the Beast might actually be. Some believe it might be a unit of measurement, such as time, distance, or a quantity. Others posit that it is a code of some sort, and if you crack it you will be able to identify the Antichrist or when and where the Apocalypse will start. There is a belief that the Number is associated with the leaders of the world. Others also, believe that the increase usage of serial numbers, RFID Badged, IP Addresses, and even 5G Towers and Vaccines are all the Mark of The Beast. As they are used to label, quantify, and track people.

The fear of the number Six Hundred and Sixty-Six is for the above reasons is officially known as hexakosioihexekontahexaphobia.