Ezekiel Chapter 9 reveals what the sign is about. To be spared from the destruction; not otherwise

When Calamity Strikes Who Is Behind It

G-d is.

And there are reasons why even the innocent or righteous may not be spared.

But it must be for the good of those who sincerely serve and worship only the one true G-d.

*Not Trinity neither Triune nor 3-in-1 G-d.

“G-d’s Wrath”

“Angel Gabriel”

He then called within my earshot [with] a loud voice, saying, “Bring near those appointed over the city, each with his weapon of destruction in his hand.”
9:1
And behold, six men were coming from the direction of the Upper Gate that faces northward, each man with his sledgehammer in his hand; but one man among them was clothed in linen, with a scribe’s slate at his loins. They came and stood by the Cooper Altar.
9:2
Then the glory of the God of Israel ascended from atop the Cherub on which it had been, going to the threshold of the Temple; and He called to the man clothed in linen, on whose loins was the scribe’s slate.
9:3
And Hashem said to him, “Pass through the midst of the city, through the midst of Jerusalem, and mark a sign on the foreheads of men who sigh and moan for all the abominations that are done within it.”
:4
Then He said to those within my earshot, “Pass through the city behind him and smite; let your eye not spare and do not have compassion.
:5
Old man, young man and maiden, children and women, shall you slay, to utter destruction; but do not approach any man upon whom is the sign. Begin from My Sanctuary!” So they began with the elders who were before the Temple.
:6
And it was while they were smiting: I remained and I fell upon my face and I cried out and said, “Alas, Lord Hashem/Elohim! Are You destroying the entire remnant of Israel as You pour out Your wrath on Jerusalem?”
:8
Then He said to me, “The iniquity of the House of Israel and Judah is very, very great — the land has been filled with blood and the city has been filled with injustice, for they have said, ‘Hashem has forsaken the land.’ and Hashem does not see.’
:9
So I, too, My eye will not spare nor will I have compassion; I have placed their way upon their head.”
:10
Just then the man clothed in linen, with the slate upon his loins, brought back and answer, saying, “I have done according to all that You have commanded me.”
:11

1 Then He called into my ears with a loud voice, saying, “Bring near those appointed over the city, and each one his weapon of destruction in his hand.”
2 And behold six men coming from the way of the upper gate, which is turned northward; each man has his sledgehammer in his hand, and one man among them clothed in linen with a scribe’s tablet on his loins, and they came and stood beside the copper altar.
3 And the glory of the God of Israel lifted itself from upon the cherub upon which it had been, to the threshold of the House, and He called to the man clothed in linen, upon whose loins was the scribe’s tablet.
4 And the Lord said to him, “Pass through the midst of the city, through the midst of Jerusalem, and you shall mark a sign upon the foreheads of the men who are sighing and moaning over all the abominations that were done in its midst.”
5 And to these, He said in my ears, “Pass through the city after him and smite; let your eye spare not and have no pity.
6 Old man, young man, and maiden, young children and women, you shall slay utterly, but to any man upon whom there is the mark you shall not draw near, and you shall commence from My sanctuary.” So they commenced from the old men who were before the House.
7 And He said to them, “Defile the House and fill the courts with the slain [and] go out,” and they went out and smote in the city.
8 And it came about when they smote, that I remained, and I fell on my face and cried out, and I said, “Alas, O Lord God, are You destroying the entire remnant of Israel when You pour out Your fury on Jerusalem?”
9 And He said to me, “The iniquity of the house of Israel and Judah is exceedingly great, and the land has become full of bloodshed, and the city has become full of perversion, for they said, ‘The Lord has forsaken the earth, and the Lord does not see.’
10 I too-My eye will not spare, neither will I have pity; I have placed their way on their head.”
11 And behold, the man clothed in linen, on whose loins was the tablet, replied with a word, saying, “I have done according to all that You have commanded me.”