ἀριθμὸς γὰρ ἀνθρώπου ἐστίν.
Maybe an "a", maybe not.
"For it is the number of man." (or "a man.")
For relevant commentary see:
https://biblehub.com/commentaries/barnes/revelation/13.htm
(There are a number of other commentary from which to choose!)
This subject is not something that will be resolved on the "Q" thread or even off the Q thread (which might be a better place for a religious discussion, which tend to be "Fractous" .) Sliding back to Q!
/slide