Eph 1:13 You, too, have heard the word of truth, the gospel of your salvation. When you believed in him you were sealed with the promised Holy Spirit. 4:30 And do not grieve the Holy Spirit of God, by whom you were sealed to the day of redemption.
According to the Ephesians letter, a person receives the seal of the Holy Spirit of God upon becoming a believer. If a person remains in faith, the seal is secured until the day of redemption. Luke 21:28 But when these things begin to happen, stand up and lift up your heads, for your redemption is drawing near. – The day of redemption refers to the events of the end-time when the world will be liberalized from this satanic world order.
Also, the angels can press the seal of God to man. The Revelation says: 7:2 And I saw another angel coming up from the rising of the sun, having a seal of the living God. And he cried with a great voice to the four angels to whom it was given to them to harm the earth and the sea, saying, 3 Do not harm the earth, nor the sea, nor the trees, until we seal the servants of our God on their foreheads 4 I heard the number of those who were sealed: 144,000. Those who were sealed were from every tribe of Israel:
9:4 And it was said to them that they should not harm the grass of the earth, nor every green thing, nor every tree, except only the men who do not have the seal of God on their foreheads.
The seal of God is a sign for the angels of God that they do not direct measures against the people with the seal. The beast, in turn, persecutes these people by killing them.