How do we know the promise of salvation through Christ is true?

The assurance of salvation comes through the sealing of the Holy Spirit, as affirmed in Ephesians 1:13-14. Once an individual believes the gospel, they are sealed by the Spirit, which acts as a promise that they are part of God's family and will receive their full inheritance. The Holy Spirit's presence in believers signifies that their salvation is secure and that God's promises are true, as He is faithful and will carry out His covenant of redemption until its completion on the last day.