John Owen: Short Definition of The Sealing of the Holy Spirit

John Owen defines how believers are sealed by the Holy Spirit by looking back at how Christ was sealed by the Holy Spirit (John 6:27). The Holy Spirit descended on Jesus (Luke 3:21), not to indwell Him, but communicate to the world around that Jesus is the foretold Messiah: God's beloved Son. Likewise, the Holy Spirit communicates to us and others our position with God.

God's sealing of believers, then, is his gracious communication of the Holy Ghost unto them, so to act his divine power in them as to enable them unto all the duties of their holy calling, evidencing them to be accepted with him, both to themselves and others, and asserting their preservation to eternal salvation. The effects of this sealing are gracious operations of the Spirit in and upon believers; but the sealing itself is the communication of the Spirit unto them.
(p. 373-347, The Holy Spirit )