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.’
There are numerous biblical references relating to the practice of sealing which contribute to understanding Paul’s exhortation. In addition, the use of seals in the Ancient World sheds light on the significance of ‘sealing’.