I'm not sure if the idol is sunday, and the text meant to say "or the name of the author of the idol of the beast" ... "for it is the number of the initials of a man"

The conflict is to wax fiercer and fiercer. Satan will take
the field and personate Christ. SpTA11 no 11 page 9

Revelation 13:13 And he (the beast-satan personating Christ Jesus Saviour) doeth great wonders, so that he maketh fire come down from heaven on the earth in the sight of men,

The enemy is preparing to deceive the whole world
by his miracle-working power. He will assume to
personate the ANGELS of LIGHT, to personate Jesus
Christ. 2nd Selected Messages p. 96

Chi-xi (IeZeus)-stigma are The Initials of the beast's name.  The beast being the man whose Greek name bears these three initials. The name of the most popular person. "Christ Jesus Saviour" or just a short while earlier (centuries) before the "j" "christ iesus saviour" or before latin, in the Greek

This image is from an old Greek Bible from Revelation 13:18, - the gif shows the capital, and small letter, of the middle Greek symbol.

of the 666 or in Greek, Chi (X), Xi (Z or I), Stigma (word names of the 666 greek symbols), which is equal numerically to 666 counting in Greek letter symbols, which 666 Ellen White mentions was made up.  There are some who say it is 616.  But actually the middle symbol is the first letter of Zeus, if you count it 666, or the first letter of Iezeus, if you read 616 as is in several Greek texts. This middle symbol is Iota, equal to 10.Zeus 60, or IeZeus 10.

This animated gif shows a capital "xi" on left of "=" and small letter "xi" on the right side of the "=" sign.

The Letter Z-...The Greeks changed the Semitic name zayin to zeta and used the sign in several forms...From one of the Greek forms the capital letter came into English unchanged. (From Compton's NewMedia)

The image above shows us that through the juxtapostion of time, and times changes, and through ignorance of accurate history, the i in caps, for iesous, became Z for Zeus. Thus the mystery of how that strange "xi" symbol was at one time the initial of "Jesus" or "Zeus", or "Iesous".

Here is the Greek word Christ

Here is the Greek word for Jesus or Iesus

The last symbol of the "Mark" is the Greek symbol for Stigma, which in the old Bible picture looks like an earing, or maybe an old ear piercing tool, perhaps, thus the symbol stigma, or "mark of a slave" was to wear an earing tag of some sort.

In old Greek mythology Zeus was the chief deity of the heathen world, whose principle weapon of warfare was the thunderbolt.  Notice Scripture.

Revelation 13:13 And he doeth great wonders, so that he maketh fire come down from heaven on the earth in the sight of men,