Historically, different kinds of seals have been used. Cylinder seals have been discovered that date back to ancient Mesopotamia, dated approximately around 3500 B.C. A package or jar was sealed with wet clay and the seal was rolled over the clay. Many shapes have been discovered ranging from cylindrical, square and round, to oval and pyramid shape.
The engraver was a common occupation in ancient times. The "signet ring," also known as a "seal" was used universally in the ancient world. Most often it was attached to a ring and worn on the finger or else it hung from the neck on a string or on the arm. The seal was covered with "ink" and then used like a present-day stamp. Those in positions of authority would seal or stamp a document, or the door of a house, or a tomb, etc. The use of a seal by men of wealth and position was common before the Christian era. Even today, one can use sealing wax on a letter, and stamp an initial or insignia into the hot wax.
In 1 Kings 21:8, Jezebel "wrote letters in Ahab's name, sealed them with his seal." Nehemiah 9:38 states: "And because of all this, we make a sure covenant and write it; Our leaders, our Levites, and our priests seal it." Nehemiah 10:1 states "Now those who placed their seal on the document were:" and all the names are listed. Once a decree or commandment was written and sealed, it was final and could not be invalidated. Because of this, the seal of the king was given to Mordecai so that he could "write a decree concerning the Jews, as you please, in the king's name, and seal it with the king's signet ring; for whatever is written in the king's name and sealed with the king's signet ring no one can revoke." (Esther 8:8)
This did not negate the first decree, but amended it. King Darius was fooled and forced into putting Daniel into a den of lions. "Then a stone was brought and laid on the mouth of the den, and the king sealed it with his own signet ring and with the signets of his lords, that the purpose concerning Daniel might not be changed." (Daniel 6:17)
With this background in mind, the verses in the New Testament regarding God's seal take on more potency. "(God) has sealed us and given us the Spirit in our hearts as a guarantee." (2 Corinthians 1:22) Similarly, in Ephesians, this same tremendous truth is repeated. "In Him you also trusted, after you heard the word of truth, the gospel of your salvation; in whom also, having believed, you were sealed with the Holy Spirit of promise, who is the guarantee of our inheritance until the redemption of the purchased possession, to the praise of His glory." (Ephesians 1:13, 14) The Amplified Bible says "stamped with the seal" and the New Century Version renders it this way: "And in Christ, God put his special mark of ownership on you by giving you the Holy Spirit that he had promised."
The seal of God is the gift of the Holy Spirit, stamped, marked, imprinted within believers, a seal that no one can revoke and that the purpose concerning God' people won't change. "And do not grieve the Holy Spirit of God, by whom you were sealed for the day of redemption." (Ephesians 4:30)