Besides those addressing that the other sheep are the Gentiles.. who are the 144,000?
In Revelation there is a motif of John hearing and then looking. Revelation 5, John is told to see the Lion of Judah, but when he looks, what he sees is the slaughtered lamb.
Both the Lion of Judah and the slaughtered lamb are Jesus. But the Jewish expectation of the Messiah leading to the overthrowing of empires that ruled them was fulfilled in an unexpected way. The Messiah’s victory was through self sacrifice.
Revelation 7, hears the number of 144,000 out of every tribe of Israel. The number being symbolic of completeness, yet when he turns and looks he sees a great multitude that he cannot count out of every nation and tribe.
That complete number of the 144,000 out of Israel is fulfilled unexpectedly as the the great multitude out of every nation. The 144,000 is the great multitude.
The concept of two classes of Christian does not exist in the Bible.