Ephesians 1:13 (NKJV), “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”.
A seal is an authoritative impression fixed to an important document. That seal testifies to its genuineness. It not only authenticates, but it also ratifies, confirms and declares the document legal. Thus it becomes the promise or assurance of its fulfillment.
The inspired writers of the New Testament symbolically apply the term to God’s gift of the holy spirit to the Church. The seal marks them as sons and heirs of God, and joint heirs with Jesus Christ. By faith, believers receive forgiveness and continue to consecrate their lives and wills to obey God are sanctified (set apart). This sanctification testifies to their being sealed by God with his holy spirit for the day of deliverance.
The seal is called the “holy spirit of promise” because it is a foretaste, a pledge, of that blessedness into which Christians shall be delivered. As Paul says (Rom. 8:23) having this first fruit of the spirit in our imperfect earthen vessel (sinful bodies) we cannot fully carry out the mind of the new spirit. So we groan within ourselves, realizing both our heavenly hopes and our human shortcomings. We longingly wait for the glorious deliverance or birth of the new creature.
The seal is distinctly felt upon the heart of the believer and is thus a testimony to himself of his divinely recognized kinship with God. However, it is also manifest to others. Our characters and behaviors become more Christ-like as the impression of the seal crystalizes. This seal testifies to the world of the wisdom, power and grace of God.
At Jesus’s death, he opened “a new and living way which He consecrated for us, through the veil, that is, His flesh”. Hebrews 10:20 (NKJV) Jesus was the first human being to be begotten to a new life with the Holy Spirit. At Pentecost, the body members of Christ also received spirit-begettal. In Old Testament times, the operations of the Holy Spirit upon men gave them skills, wisdom, and visions, but it did not begin a new life. The Old Testament did not mention the sealing because none were sealed with the promise of a new heavenly life.