Question: Where in the Bible is the Mark of the Beast, 666?
Answer: 666, the Mark of the Beast, is found in Revelation 13:18. There is also an obscure, likely unrelated reference to the same number in the Old Testament in 2 Samuel 2:31, so I have included that below as well.
"Here is wisdom. Let him who has understanding calculate the number of the beast, for the number is that of a man; and his number is six hundred and sixty-six." Revelation 13:18.
"But the servants of David had struck down many of Benjamin and Abner’s men, so that three hundred and sixty men died." 2 Samuel 2:31.