For him hath God the Father sealed - John 6:27

The seal is the mark of authentication. The Book of Esther often refers to the importance of the royal seal as giving validity and authenticity to documents to which it was appended. So at the waters of Jordan God authenticated our Lord; first by the voice that spoke from heaven, and secondly by the holy anointing that came upon His head, setting Him apart for holy service. What the Father did for His Son, He does for His sons. "He that stablisheth us with you into Christ, and anointed us, is God, who also sealed us." In other words, God waits to authenticate us to ourselves and to the world, as His beloved children, in whom He is well pleased.
The conditions of sealing. - In the case of our Lord there was entire subjection to the Father's will, although it involved His leaving the blessed home of Nazareth and identifying Himself with the sins and sorrows of men, by baptism in waters where they had confessed their sins. We, too, must be prepared to obey utterly, even to death.
The agent of sealing. - The Spirit descended and abode upon Him; He was filled with the Spirit, and returned in His power to Galilee. We, too, are sealed by the Holy Spirit of promise; who stamps us with the die of our Saviour's image and superscription. Simultaneously with His gracious work upon us, we may detect His loving voice within us, witnessing with our spirits that we are children of God.
The effect of sealing. - Secrecy, safety, and assurance. Secrecy, Son 4:12. Safely, Mat 27:65-66. Assurance, Rom 8:15, Rom 8:16, Rom 8:17. There is also a daily assimilation, though we know it not, to the glorious likeness of our Lord; so that those who see us bear witness that His name is on us.