Istvan:
So who are the 144,000 in the book of revelation?
First, Istvan, it is good to hear from you! I hope all is going well.
Second, to give my final answer before I ramble, I don't exactly know (and I believe I am not alone).
The straight-forward Biblical answer is that the 144,000 are those "who have not defiled themselves with women, for they are virgins. It is these who follow the Lamb wherever he goes. These have been redeemed from mankind as firstfruits for God and the Lamb, and in their mouth no lie was found, for they are blameless." (Rev. 14:4-5). We also see they know a song that "no one could learn ... except the 144,000." In chapter 7 they are associated with the sons of Israel: Judah, Reuben, Gad, Asher, Naphtali, Manasseh, Simeon, Levi, Issachar, Zebulun, Joseph, and Benjamin. But we know this is not the nationalistic Israel for Levi and Joseph are included and Ephraim and Dan are missing. And we know this is not the biological Israel because Manasseh is included (interestingly it was Ephraim, the younger, who was placed ahead of Manasseh, Genesis 48).
What is my conclusion? The 144,000 are the "chosen race, a royal priesthood, a holy nation, a people for His own possession." These titles are found in the OT for Israel. However, in the NT Peter uses it for God's elect--both Jew and Gentile (1 Peter 2:9-10).
For me it comes down to this, there are two siblings in the Family of God--the natural born and the adopted. Both are heirs to the promises. If the 144,000 is only the natural born sibling (who is currently not holding up his seat in the Family), why is the adopted sibling not able to learn the song of Revelation? I believe this flies in the face of the established doctrines throughout the letters of Paul (something Dispensationalists do regularly).
(Did I say yet, welcome back, Istvan?) 