WCF 18.2.
This certainty is not a bare conjectural and probable persuasion grounded upon a fallible hope; (Heb. 6:11, 19) but an infallible assurance of faith founded upon the divine truth of the promises of salvation, (Heb. 6:17–18) the inward evidence of those graces unto which these promises are made, (2 Pet. 1:4–5, 10–11, 1 John 2:3. 1 John 3:14, 2 Cor. 1:12) the testimony of the Spirit of adoption witnessing with our spirits that we are the children of God, (Rom. 8:15–16) which Spirit is the earnest of our inheritance, whereby we are sealed to the day of redemption. (Eph. 1:13–14, Eph. 4:30, 2 Cor. 1:21–22)