There is a clear distinction made between the two groups of Tribulation saints. The account shows the first group consisting of 144,000 being sealed/marked. (Rev. 7:2-8) Since Christians are sealed upon obedience to the gospel message they are not numbered in the 144,000. And verse 9 specifically references that it is after the command to seal the 144,000 that a multitude
that cannot be numbered is mentioned.
My thought on this is the tribes of the children of Israel are those who remain blind to the reality that Jesus was in fact their Messiah. And this occurs by the providence of God for a reason; of which only He knows the full. The 144,000 Jews alive at the time of the end, will be the descendants of those who have not wavered in their commitment to God in obedience to the OT mandates. And while patiently awaiting their Messiah will have reinstated the sacrificial system established by God. It may be that the false prophet will cause fire to come down from heaven to initiate the sacrificial system in the newly constructed third temple. If so, this counterfeit action will deceive many into worshipping the Antichrist. (Rev. 13:13)
What many fail to realize is the Book of Revelation is in fact the record of the revealing/unveiling of the Lord Jesus Christ. Paul mentions that the veil of blindness upon some of the nation of Israel is a mystery only known to God. And that it will occur once the time of the Gentiles is fulfilled. And we know this fulfillment has not yet come. Many are still hearing and believing the gospel message. However, one thing is for sure. And that is they, too, will realize along with the very last Gentile saved that the coming of their Messiah is in fact His second coming.
The following website has some relevant information:
https://israelmyglory.org/article/the-restoration-of-israel-part-two/
Rev 7:2-8
2 And I saw another angel ascending from the east, having the seal of the living God: and he cried with a loud voice to the four angels, to whom it was given to hurt the earth and the sea,
3 Saying, Hurt not the earth, neither the sea, nor the trees, till we have sealed the servants of our God in their foreheads.
Those sealed numbered 144,000:
4 And I heard the number of them which were sealed: and there were sealed an hundred and forty and four thousand of all the tribes of the children of Israel.
5 Of the tribe of Juda were sealed twelve thousand. Of the tribe of Reuben were sealed twelve thousand. Of the tribe of Gad were sealed twelve thousand.
6 Of the tribe of Aser were sealed twelve thousand. Of the tribe of Nepthali were sealed twelve thousand. Of the tribe of Manasses were sealed twelve thousand.
7 Of the tribe of Simeon were sealed twelve thousand. Of the tribe of Levi were sealed twelve thousand. Of the tribe of Issachar were sealed twelve thousand.
8 Of the tribe of Zabulon were sealed twelve thousand. Of the tribe of Joseph were sealed twelve thousand. Of the tribe of Benjamin were sealed twelve thousand.
Rev 7:9
"
After this I beheld, and, lo, a great
multitude, which no man could number, of all nations, and kindreds, and people, and tongues, stood before the throne, and before the Lamb, clothed with white robes, and palms in their hands;"