The Spirit Seals a Child of God.
“So the Regal Spirit seals every believer in the possession of all the blessings of redemption. In this case, it is the seal of the King of kings that cannot be broken. Beyond making you certain at the present moment that you have been redeemed, the Holy Spirit seals you in the permanent possession of your salvation. For “having believed, you were marked in him with a seal, the promised Holy Spirit, who is a deposit guaranteeing our inheritance” until the day of Christ’s return (Eph. 1:13b, 14a). It is a settled fact. His sealing work cannot be undone — all “to the praise of his glory” (Eph. 1:14b).”
The Wind Blows Where it Wills: The Wondrous Work of the Holy Spirit Within the Believer by O. Palmer Robertson.