We read in, Revelation 7:2-3;
‘Then I saw another angel coming up from the east, having the seal of the living God. He called out in a loud voice to the four angels who had been given power to harm the land and the sea: Do not harm the land or the sea or the trees until we put a seal on the foreheads of the servants of our God.’
This is a parallel to Mt 24:31, “His angels shall gather together His elect from the four winds.”
God’s love is such, that He cannot do anything in the way of judgment until His people are secured from hurt. The angel of the Lord said to Lot when he came to destroy Sodom and Gomorrah and to save him in Genesis 19:22;
“But flee there quickly, because I cannot do anything until you reach it.” (That is why the town was called Zoar.)
The design of sealing the persons referred to seems to have been to secure their safety in the impending calamities. Thus, the winds were held back until those who were to be sealed could be designated, and then they were to be allowed to sweep over the earth.
There is also an allusion here to Ezekiel 9:4-6:
“Go throughout the city of Jerusalem and put a mark on the foreheads of those who grieve and lament over all the detestable things that are done in it.” As I listened, he said to the others, “Follow him through the city and kill, without showing pity or compassion. Slaughter old men, young men and maidens, women and children, but do not touch anyone who has the mark. Begin at my sanctuary.” So they began with the elders who were in front of the temple.””
This mark is the witness of the Holy Spirit printed in the hearts of believers. And the Lord would not let His people to be afflicted before they were marked. And of those who are sealed by the Holy Spirit, the seal must be on the forehead.
The seal was also a mark of ownership. The seal of God on the forehead would mark those sealed at God’s. The seal mark on the forehead would be visible to everyone; therefore the seal in the forehead has been understood to be the public confession and profession of Christ.
Christ has the Spirit, with His gifts and graces, without measure, by which the saints are sealed unto the day of redemption; and moreover has likewise the seal of the book of life, or of eternal election, in His hands; the elect are chosen in Him, and the Book of Life, in which their names are written to eternal life, is in His keeping, and is therefore called the Lamb’s Book of Life.