Ephesians 1:13-14, In [Christ] 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.
Here, Paul makes known to all Christians where assurance of salvation lies. It lies not in your works or human understanding, but in the Holy Spirit. If you believe in Christ, you are sealed with the Holy Spirit; He alone keeps you in Christ.
In Stevie Wonder’s love song, Signed, Sealed, Delivered, he sings the famous lyrics, “Signed, sealed, delivered, I’m yours!” Signed, sealed, delivered, you are God’s! By faith in Christ, the Holy Spirit has signed your salvific fate, sealed you in Christ, and delivered you to God the Father. Like a letter in an envelope, the Holy Spirit has placed you in Christ, His seal upon you, and delivered you into the hands of God the Father who sees the seal of the Holy Spirit upon you and accepts you.
By faith, you have the Holy Spirit, who is the guarantee of your inheritance of eternal life. If you ever doubt that you have salvation, ask yourself this question: “Do I believe in Christ?” If the answer is “yes,” then you have the Holy Spirit, who alone enables belief (1 Corinthians 12:3), and you therefore have Him as the guarantee of your salvation. If any sins trouble you, repent and trust in the promise of Christ. If you are baptised, you know for certain you have the Holy Spirit, and therefore the guarantee of salvation.