The seal of the Spirit and assurance
Ephesians 1.13-14 is a crucial text that explains how the Holy Spirit provides assurance of salvation for the believer.
‘And you also were included in Christ when you heard the word of truth, the gospel of your salvation. Having believed, you were marked in him with a seal, the promised Holy Spirit, who is a deposit guaranteeing our inheritance until the redemption of those who are God's possession — to the praise of his glory.’