The letter for chapter Seven is "Zayin" meaning sustenance and struggle. This is not implying that the number seven does not also mean perfection. So how does "Zayin" relate to Ch. Seven?
Obviously this is a time of struggle because the seals are broken and the wrath of G-d is being unleashed. Some believe that the first part of the tribulation is the wrath of man. Now we are entering the wrath of G-d against those who are destroying the earth. For the wrath of the L-rd can even be seen in the celestial bodies. (sun, stars and moon)
Revelation 7:1-3 "And after these things I saw four angels standing on the four corners of the earth, holding the four winds of the earth, that the wind should not blow on the earth, nor on the sea, nor on any tree. And I saw another angel ascending from the east, having the seal of the living G-d: and he cried with a loud voice to the four angels, to whom it was given to hurt the earth and the sea, Saying, Hurt not the earth, neither the sea, nor the trees, till we have sealed the servants of our G-d in their foreheads. (KJV)
Bible scholars are at odds about the time that chapter 7 occurs. Some believe that it will occur after the 6th seal (the great earthquake) others believe that it will occur before the 1st seal is broken.
We see 4 angels at the 4 corners of the earth holding back the 4 winds so that no wind would blow on the earth, sea, or trees. Here we see our G-d is sovereign over the elements.
Another angel from the east comes with the seal of G-d. He cries to the other angels to wait until he has sealed the servants of G-d.
Revelation 7:4-8 "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. 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. Of the tribe of Aser were sealed twelve thousand. Of the tribe of Nepthalim were sealed twelve thousand. Of the tribe of Manasses were sealed twelve thousand. 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. 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. (KJV)
There will be 12,000 sealed from each of the 12 tribes of Israel. The total of this group is 144,000.
When the 12 tribes are listed in various Bible passages the lists will vary. Jacob whose name was later changed to Israel by G-d had 12 sons to 4 women.
Joseph had 2 sons Ephraim and Manasseh. Israel said that they would inherit with His sons. So in some lists you will not see Joseph but Ephraim and Manasseh listed. Also Levi is often not listed because they do not inherit land, as they are the priestly tribe. Their inheritance is the L-rd. (Deut 9:10)
In the list given in Chapter 7, Joseph is listed with his son Manasseh. The tribe of Dan is missing as well as the tribe of Ephraim.
Why? Some believe that Dan is missing because they think the Anti-Messiah will come from the tribe of Dan they use Genesis 49:17 as their proof text.
Others believe Dan was one of the first to go into idolatry; it was a small tribe and was possibly classified with Naphtali, because they share the same mother.
I think it may have something to do with this passage in
Deuteronomy 29:18-21"Lest there should be among you man, or woman, or family, or tribe, whose heart turneth away this day from the L-RD our G-d, to go and serve the gods of these nations; lest there should be among you a root that beareth gall and wormwood; And it come to pass, when he heareth the words of this curse, that he bless himself in his heart, saying, I shall have peace, though I walk in the imagination of mine heart, to add drunkenness to thirst: The L-RD will not spare him, but then the anger of the L-RD and his jealousy shall smoke against that man, and all the curses that are written in this book shall lie upon him, and the L-RD shall blot out his name from under heaven. And the L-RD shall separate him unto evil out of all the tribes of Israel, according to all the curses of the covenant that are written in this book of the law:" (KJV)
Both the tribes of Dan and Ephraim participated in introducing idolatry into Israel.
(See Judges 18:2,30-31; 1 Kings 11:26;12:28-30 Hosea 4:17)
It is believed that they are sealed so that no harm will come to them
and they will go out preaching the gospel to the entire world. It appears
their ministry may be limited to the first 3 1/2 years of the tribulation
period. (More on them in chapter 14).