A seal, like a signature, is used to validate a document. In ancient times it was a stamp pressed onto soft wax or clay to show authenticity or ownership, having the authority of its owner behind it.
The seal of God is a sign of God’s ownership and protection of His people. Paul describes a sealing in connection with conversion and reception of the gift of the Holy Spirit. He calls this gift a “deposit” or “down payment” given to all believers as an assurance of the complete redemption and future inheritance they will receive when Jesus comes.
The book of Revelation pictures another sealing just prior to the second advent. This final seal is given to the 144,000 at the time of the outpouring of the Holy Spirit in the latter rain. They have God’s name (or signature) written in their foreheads. Through the Holy Spirit’s work in their lives they come to reflect God’s character.
The seal is given to true worshipers of God while the mark is given to worshipers of the beast. The seal is given only in the forehead, indicating a definite choice of the mind to worship God in the way that He has commanded. The mark, on the other hand, is given either in the forehead or in the hand. This means that people may worship the beast for one of two reasons. Either in their minds they agree with it, thinking that they are truly worshiping God, or else they don’t agree with it but they go along because they are afraid of the serious consequences of not conforming: being unable to buy or sell and eventually being killed (Rev. 13:17, Rev. 13:15) .
“Those who are uniting with the world are receiving the worldly mold and preparing for the mark of the beast. Those who are distrustful of self, who are humbling themselves before God and purifying their souls by obeying the truth — these are receiving the heavenly mold and preparing for the seal of God in their foreheads”. — Ellen G. White, Testimonies for the Church , vol. 5, p. 216.
Source: Daily Sabbath School Lessons