Sealed for God on the forehead?

After this I saw four angels standing at the four corners of the earth, holding back the four winds of the earth, that no wind might blow on earth or sea or against any tree. Then I saw another angel ascending from the rising of the sun, with the seal of the living God, and he called with a loud voice to the four angels who had been given power to harm earth and sea, saying, “Do not harm the earth or the sea or the trees, until we have sealed the servants of our God on their foreheads.”

Revelation 7:1-3

Why and how are God’s servants sealed with God’s seal and why on their foreheads?

When reading Revelation we should remember that we are dealing with Apocalyptic literature and the images and numbers should be understood symbolically, not literally.  

The vision is of four mighty angels holding back destructive winds.  The winds, or if you prefer the winds of woe*, are the seals of Revelation Ch. 6.

They are the winds of hardship, war, persecution, famine and ultimately of God’s final judgment (cf. Revelation 6:12-17) which will blow on the earth.

But the mighty angels are holding them back.