Rev 7:1  "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." 

In this verse there are three fours: four angels; four corners; and four winds.  Four is associated with the bible subject of the work of the Holy Spirit.  For this association see the four beasts; the four living creatures and their four faces; the four horses; the four chariots representing the four spirits of the heavens; the four rivers flowing out of the garden of Eden; the four gospels; the four wheels going wherever the spirit led them; and the four Hebrew and Greek words translated into the English word Spirit. 

The word "angel" literally means messenger.  Thus the four angels are spiritual messengers bringing their message to those appointed for it.  As we shall see in v.2 it was given to the angels to hurt the earth and the sea.  In Rev. chapter 9 we see these four angels bound in the great river Euphrates and they were to slay the third part of men.  We will have much more to say about these four angels and their work when we get to the 9th chapter.           

The word "wind" is often associated with the "spirit" in the scriptures.  The four winds are used to indicate the entirety of the earth: Mark 13:27 "And then shall he send his angels, and shall gather together his elect from the four winds, from the uttermost part of the earth to the uttermost part of heaven."  Furthermore as the winds are directed of God so the Lord's people are directed by the Spirit: Zec. 2:6 "Ho, ho, come forth, and flee from the land of the north, saith the LORD: for I have spread you abroad as the four winds of the heaven, saith the LORD."  In Ezekiel the four winds were used in a way to indicate the work of the Holy Spirit in giving life: Ezek. 37:9 "Then said he unto me, Prophesy unto the wind, prophesy, son of man, and say to the wind, Thus saith the Lord GOD; Come from the four winds, O breath, and breathe upon these slain, that they may live."         

The phrase "four corners of the earth" is used to indicate the four quarters of the earth or the totality of the earth.  We read in Is. 11:12 "And he shall set up an ensign for the nations, and shall assemble the outcasts of Israel, and gather together the dispersed of Judah from the four corners of the earth."  Both the Greek word in Rev. 7:1 and the Hebrew word in Is. 11:12 that are translated "corners" literally means quarters. 

The "earth" is sometimes used to indicate the dwelling place of God's people:

          1.  Ps. 25:13 "His soul shall dwell at ease; and his seed shall inherit the earth."

          2.  Ps. 37:9 "For evildoers shall be cut off: but those that wait upon the LORD, they shall inherit the earth."

          3.  Ps. 37:11 "But the meek shall inherit the earth; and shall delight themselves in the abundance of peace."

          4.  Ps. 37:22 "For such as be blessed of him shall inherit the earth; and they that be cursed of him shall be cut off."

          5.  Matt. 5:5 "Blessed are the meek: for they shall inherit the earth."         

The word sea is sometimes used to indicate the "sea" of humanity: 

          1.  Is. 60:5 "Then thou shalt see, and flow together, and thine heart shall fear, and be enlarged; because the abundance of the sea shall be converted unto thee, the forces of the Gentiles shall come unto thee."

          2.  Jere. 51:42 "The sea is come up upon Babylon: she is covered with the multitude of the waves thereof."

          3.  Ezek 26:16 "Then all the princes of the sea shall come down from their thrones, and lay away their robes, and put off their broidered garments: they shall clothe themselves with trembling; they shall sit upon the ground, and shall tremble at every moment, and be astonished at thee."

          4.  Dan. 7:3 "And four great beasts came up from the sea, diverse one from another."

          5.  Zec. 14:8 "And it shall be in that day, that living waters shall go out from Jerusalem; half of them toward the former sea, and half of them toward the hinder sea: in summer and in winter shall it be."

          6.  Rev. 13:1 "And I stood upon the sand of the sea, and saw a beast rise up out of the sea, having seven heads and ten horns, and upon his horns ten crowns, and upon his heads the name of blasphemy." 

The "tree" or "trees" are used in a lot of different ways in the scriptures.  However, it is almost always indicative of things which have life.  The usage of tree in verse 1 seems to indicate singularity as opposed to the use of trees which would indicate plurality.  Thus that the wind would not blow on any tree is indicative of no individual life being hurt while the sealing process was going on.  Some had to be sealed before anyone was hurt.

Rev 7:2, 3  "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."

Bible directions all have significance in the scriptures.  For instance the direction of east is associated with sin:

          1.  When God drove man out of the garden because of sin, He placed at the east of Eden Cherubims and a flaming sword that kept the way of the tree of life.

          2.  Cain slew his brother on the east of Eden.

          3.  It was in the east, in the land of Shinar that man built the tower of Babel in rebellion against the commandment of God.

          4.  When Abraham made an altar to worship the Lord, he had on east of him Hai (which means heap) and on the west he had Bethel (which means house of God).

          5.  When Lot separated from Abram he journeyed east to Sodom.

          6.  In Pharaoh's dream the east wind blasted the seven thin ears of corn.

          7.  The east wind brought the locusts of judgment upon the land of Egypt.

The direction of west is associated with righteousness:

          1.  When Abram built an altar for worship he had Bethel (house of God) on the west.

          2.  A mighty west wind took away the locusts of judgment upon the land of Egypt.

          3.  Is. 59:19 "So shall they fear the name of the LORD from the west, and his glory from the rising of the sun. When the enemy shall come in like a flood, the Spirit of the LORD shall lift up a standard against him."

The angel was ascending from the east, thus he was headed in the west direction or the direction of righteousness.  The fact that he was ascending also indicates he was going in the direction of righteousness.  The angel had the seal of God.  This seal was different from the seven seals on the book of salvation.  This seal was an authenticating seal.  Kings and others had seals, usually on their rings, by which they authenticated that the document or thing on which they impressed the seal was a valid document or thing and represented the authority of the king:

          1.  1 Kings 21:8 "So she wrote letters in Ahab's name, and sealed them with his seal, and sent the letters unto the elders and to the nobles that were in his city, dwelling with Naboth."

          2.  Neh. 9:38 "And because of all this we make a sure covenant, and write it; and our princes, Levites, and priests, seal unto it."

          3.  Est. 3:12 "Then were the king's scribes called on the thirteenth day of the first month, and there was written according to all that Haman had commanded unto the king's lieutenants, and to the governors that were over every province, and to the rulers of every people of every province according to the writing thereof, and to every people after their language; in the name of king Ahasuerus was it written, and sealed with the king's ring."

          4.  Est. 8:8 "Write ye also for the Jews, as it liketh you, in the king's name, and seal it with the king's ring: for the writing which is written in the king's name, and sealed with the king's ring, may no man reverse."

          5.  Est. 8:10 "And he wrote in the king Ahasuerus' name, and sealed it with the king's ring, and sent letters by posts on horseback, and riders on mules, camels, and young dromedaries:"

          6.  Dan. 6:17 "And a stone was brought, and laid upon the mouth of the den; and the king sealed it with his own signet, and with the signet of his lords; that the purpose might not be changed concerning Daniel."

The seal referenced in verse 2 is the seal of God indicating it came from God and is a symbol of the authority of God and that the one sealed by it is authentic and is authorized by God.  It is the servants of God and only the servants of God that were sealed in their forehead.  We read in 2 Cor. 1:21 "Now he which stablisheth us with you in Christ, and hath anointed us, is God; 22 Who hath also sealed us, and given the earnest of the Spirit in our hearts."  Notice that God did the sealing and it was the servants (members of the church at Corinth and the one who preached to them) that were sealed. 

We also read in Eph. 1:13 "In whom ye also trusted, after that ye heard the word of truth, the gospel of your salvation: in whom also after that ye believed, ye were sealed with that holy Spirit of promise, 14 Which is the earnest of our inheritance until the redemption of the purchased possession, unto the praise of his glory."  Notice that the ones sealed were sealed after they heard the word of truth, the gospel of their salvation, and after that they believed the truth and trusted in Christ.  The sealing was a testimony to them that they were of God and were the children of promise.  This is the earnest of their inheritance.  The sealing didn't make them children of God but testifies to them through their experiences and belief that they are children of God and servants of God.

The sealing in the forehead is simply indicating that they were sealed in their minds by the Holy Spirit of promise.