Part III: Questions about Salvation
105. Q. How does the Holy Spirit seal believers?
A. He comes to live within them to guarantee that they will receive the wonders God has promised those who love Him.
(Click through to read scriptural proof.)
You, however, are not in the flesh but in the Spirit, if in fact the Spirit of God dwells in you. Anyone who does not have the Spirit of Christ does not belong to him.  But if Christ is in you, although the body is dead because of sin, the Spirit is life because of righteousness.  If the Spirit of him who raised Jesus from the dead dwells in you, he who raised Christ Jesus from the dead will also give life to your mortal bodies through his Spirit who dwells in you.
In him you also, when you heard the word of truth, the gospel of your salvation, and believed in him, were sealed with the promised Holy Spirit,  who is the guarantee of our inheritance until we acquire possession of it, to the praise of his glory.
And do not grieve the Holy Spirit of God, by whom you were sealed for the day of redemption.
…who saved us and called us to a holy calling, not because of our works but because of his own purpose and grace, which he gave us in Christ Jesus before the ages began,
…and who has also put his seal on us and given us his Spirit in our hearts as a guarantee.
Scripture quoted from the English Standard Version.