This post is part of a series on eternal security. If you’d like some background, you can read the introductory post: eternal-security-why-or-why-not.
God gives the believer His Holy Spirit. A genuine believer can know he or she is eternally secure because of Ephesian 1:14. You become a genuine believer by hearing the word truth, which is the gospel, and believing the truth. This provides you eternal salvation.
As soon as you believe, God seals you with His Holy Spirit so you can feel secure. God personally guarantees your inheritance until you receive it. If God is the guarantor, can there be any doubt He will keep His promise?
In him we have obtained an inheritance, having been predestined according to the purpose of him who works all things according to the counsel of his will, so that we who were the first to hope in Christ might be to the praise of his glory. 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.—Ephesians 1:11-14