The following is an excerpt from The MacArthur New Testament Commentary on Ephesians 1 .

You were sealed in Him with the Holy Spirit of promise, who is given as a pledge of our inheritance (Ephesians 1:13–14)  

As one means of guaranteeing His promises to those who have received Jesus Christ, God has sealed [them] in Him with the Holy Spirit of promise. Every believer is given the very Holy Spirit of God the moment he trusts in Christ. “You are not in the flesh but in the Spirit, if indeed the Spirit of God dwell-s in you,” Paul declares (Rom. 8:9a). Conversely, he goes on to say, “If anyone does not have the Spirit of Christ, he does not belong to Him” (v. 9b). Incredibly, the body of every true Christian is actually “a temple of the Holy Spirit who is in [him]” (1 Cor. 6:19).

When a person becomes a Christian, the Holy Spirit takes up residence in his life. Life in Jesus Christ is different because the Spirit of God is now within. He is there to empower us, equip us for ministry, and function through the gifts He has given us. The Holy Spirit is our Helper and Advocate. He protects and encourages us. He also guarantees our inheritance in Jesus Christ. “The Spirit Himself bears witness with our spirit that we are children of God, and if children, heirs also, heirs of God and fellow heirs with Christ” (Rom. 8:16–17). The Spirit of God is our securing force, our guarantee.

The sealing of which Paul speaks here refers to an official mark of identification that was placed on a letter, contract, or other important document. The seal usually was made from hot wax, which was placed on the document and then impressed with a signet ring. The document was thereby officially identified with and under the authority of the person to whom the signet belonged.

That is the idea behind our being sealed in Him [Christ] with the Holy Spirit of promise. The seal of God’s Spirit in the believer signifies four primary things: security, authenticity, ownership, and authority.

Security. In ancient times the seal of a king, prince, or noble represented security and inviolability. When Daniel was thrown into the lion’s den, King Darius, along with his nobles, placed their seals on the stone placed over the entrance to the den, “so that nothing might be changed in regard to Daniel” (Dan. 6:17). Any person but the king who broke or disturbed that seal would likely have forfeited his life. In a similar way the tomb where Jesus was buried was sealed. Fearing that Jesus’ disciples might steal His body and falsely claim His resurrection, the Jewish leaders obtained Pilate’s permission to place a seal on the stone and to guard it with soldiers (Matt. 27:62–66).

In an infinitely greater way, the Holy Spirit secures each believer, marking him with His own inviolable seal.

Authenticity. When King Ahab tried unsuccessfully to get Naboth to sell or trade his vineyard, Queen Jezebel volunteered to get the vineyard her way. “So she wrote letters in Ahab’s name and sealed them with his seal” and sent the letters to various nobles who lived in Naboth’s city, demanding that they arrange false accusations of blasphemy and treason against him. The nobles did as they were instructed, and Naboth was stoned to death because of the false charges. The king then simply confiscated the vineyard he had so strongly coveted (1 Kings 21:6–16). Despite the deceptions contained in the letters Jezebel sent, the letters themselves were authentically from the king, because they were sent with his approval and marked with his seal. The seal was his signature.

When God gives us His Holy Spirit, it is as if He stamps us with a seal that reads, “This person belongs to Me and is an authentic citizen of My divine kingdom and member of My divine family.”

Ownership. While Jerusalem was under siege by Nebuchadnezzar and Jeremiah was under arrest by King Zedekiah for prophesying against the king and the nation, the Lord gave special instructions to His prophet. Jeremiah was told to buy some land in Anathoth for which he had redemption rights. The contract was agreed on, and the stipulated payment was made in the court of the palace guard before the required number of witnesses. In the presence of the witnesses the deed was signed and sealed, establishing Jeremiah as the new legal owner of the property (Jer. 32:10).

When the Holy Spirit seals believers, He marks them as God’s divine possessions, who from that moment on entirely and eternally belong to Him, The Spirit’s seal declares the transaction of salvation as divinely official and final.

Authority. Even after Haman had been hanged for his wicked plot to defame and execute Mordecai, Queen Esther was distressed about the decree that Haman had persuaded King Ahasuerus to make that permitted anyone in his kingdom to attack and destroy the Jews. Because the king could not even himself revoke the decree that was marked with his own seal, he issued and sealed another decree that permitted and even encouraged the Jews to arm and defend themselves (Esther 8:8–12). In both cases the absolute authority of the decrees was represented in the king’s seal. Those who possessed the sealed decree of the king had the king’s delegated authority set forth in the decree.

When Christians are sealed with the Holy Spirit they are delegated to proclaim, teach, minister, and defend God’s Word and His gospel with the Lord’s own authority.