The 144,000
Whether the sealed are Israel and Dan's absence matters, or the number stands for the whole church.
Revelation 7:1-8
1After this, I saw four angels standing at the four corners of the earth, holding the four winds of the earth, so that no wind would blow on the earth, or on the sea, or on any tree.
2I saw another angel ascend from the sunrise, having the seal of the living God. He cried with a loud voice to the four angels to whom it was given to harm the earth and the sea,
3saying, “Don’t harm the earth, neither the sea, nor the trees, until we have sealed the bondservants of our God on their foreheads!”
4I heard the number of those who were sealed, one hundred forty-four thousand, sealed out of every tribe of the children of Israel:
5of the tribe of Judah twelve thousand were sealed, of the tribe of Reuben twelve thousand, of the tribe of Gad twelve thousand,
6of the tribe of Asher twelve thousand, of the tribe of Naphtali twelve thousand, of the tribe of Manasseh twelve thousand,
7of the tribe of Simeon twelve thousand, of the tribe of Levi twelve thousand, of the tribe of Issachar twelve thousand,
8of the tribe of Zebulun twelve thousand, of the tribe of Joseph twelve thousand, and of the tribe of Benjamin twelve thousand were sealed.
What each tradition says
Rabbinic / Midrashic
The Jewish Encyclopedia says the 144,000 servants of God are 12,000 from each of the twelve tribes of Israel. Dan is excluded as an idolater, and Levi takes his place along with the two sons of Joseph.
Jewish Encyclopedia (1905), article "Revelation (Book of)", The Jewish Encyclopedia (Funk & Wagnalls, 1901-1906) — "The Main Apocalypse: Opening of the Seventh Seal" (on vii. 1-8)
Patristic
Victorinus states that the number of those who will believe, from the Jews and from the nations, is a great multitude that no man was able to number. He says this is what is shown.
Victorinus of Pettau, Commentary on the Apocalypse of the Blessed John — ANF vol. 7 (ed. Roberts & Donaldson) — From the seventh chapter (on Rev. vii. 3-4)
Catholic
Witham explains that the determinate number need only mean a great number of Jews converted and saved. He adds that the saved taken from among the Gentiles of all nations was much greater, described as a great multitude no one could number.
George Leo Haydock (citing Witham), George Haydock's Catholic Bible Commentary (1859) — On Revelation 7:4 (Witham's note)
Orthodox
No sourced position found. Checked CCEL NPNF 2nd series (Cyril of Jerusalem's Catechetical Lectures, npnf207; Gregory of Nyssa, npnf205) — no treatment of Rev 7; archive.org holds only modern copyrighted English translations of the Eastern commentators on Revelation (Andrew of Caesarea, "Commentary on the Apocalypse", 2012 upload — lending-restricted; Oecumenius, 2006 scan — restricted). No public-domain English text by an Orthodox writer on Rev 7:1-8 found.
Reformed / Protestant
Matthew Henry notes some take the 144,000 as a select number of Jews reserved for mercy at Jerusalem's destruction; others think that time past, applying it to God's chosen remnant in the world. He says it seems more proper to understand them as the remnant of that people reserved according to election of grace, a definite number for an indefinite.
Matthew Henry, An Exposition of the Old and New Testament (Matthew Henry's Commentary), vol. VI (Acts to Revelation) — On Revelation 7:4-8 ("The Sealing of the Servants of God")
Evangelical
John Wesley says the hundred forty-four thousand are either those out of all mankind who had been most eminently holy, or the most holy out of the twelve tribes of Israel. He notes this is the same group mentioned in Revelation 7:4.
John Wesley, John Wesley's Explanatory Notes on the New Testament — On Revelation 14:1-5 — gloss on "an hundred forty-four thousand," referring back to Rev 7:4