1689 London Baptist Confession of Faith--Chapter 18: Of the Assurance of Grace and Salvation

1._____ Although temporary believers, and other unregenerate men, may vainly deceive themselves with false hopes and carnal presumptions of being in the favour of God and state of salvation, which hope of theirs shall perish; yet such as truly believe in the Lord Jesus, and love him in sincerity, endeavouring to walk in all good conscience before him, may in this life be certainly assured that they are in the state of grace, and may rejoice in the hope of the glory of God, which hope shall never make them ashamed.
( Job 8:13, 14; Matthew 7:22, 23; 1 John 2:3; 1 John 3:14, 18, 19, 21, 24; 1 John 5:13; Romans 5:2, 5 )

2._____ This certainty is not a bare conjectural and probable persuasion grounded upon a fallible hope, but an infallible assurance of faith founded on the blood and righteousness of Christ revealed in the Gospel; and also upon the inward evidence of those graces of the Spirit unto which promises are made, and on the testimony of the Spirit of adoption, witnessing with our spirits that we are the children of God; and, as a fruit thereof, keeping the heart both humble and holy.
( Hebrews 6:11, 19; Hebrews 6:17, 18; 2 Peter 1:4, 5, 10, 11; Romans 8:15, 16; 1 John 3:1-3 )

3._____ This infallible assurance doth not so belong to the essence of faith, but that a true believer may wait long, and conflict with many difficulties before he be partaker of it; yet being enabled by the Spirit to know the things which are freely given him of God, he may, without extraordinary revelation, in the right use of means, attain thereunto: and therefore it is the duty of every one to give all diligence to make his calling and election sure, that thereby his heart may be enlarged in peace and joy in the Holy Spirit, in love and thankfulness to God, and in strength and cheerfulness in the duties of obedience, the proper fruits of this assurance; -so far is it from inclining men to looseness.
( Isaiah 50:10; Psalms 88; Psalms 77:1-12; 1 John 4:13; Hebrews 6:11, 12; Romans 5:1, 2, 5; Romans 14:17; Psalms 119:32; Romans 6:1,2; Titus 2:11, 12, 14 )

4._____ True believers may have the assurance of their salvation divers ways shaken, diminished, and intermitted; as by negligence in preserving of it, by falling into some special sin which woundeth the conscience and grieveth the Spirit; by some sudden or vehement temptation, by God's withdrawing the light of his countenance, and suffering even such as fear him to walk in darkness and to have no light, yet are they never destitute of the seed of God and life of faith, that love of Christ and the brethren, that sincerity of heart and conscience of duty out of which, by the operation of the Spirit, this assurance may in due time be revived, and by the which, in the meantime, they are preserved from utter despair.
( Canticles 5:2, 3, 6; Psalms 51:8, 12, 14; Psalms 116:11; Psalms 77:7, 8; Psalms 31:22; Psalms 30:7; 1 John 3:9; Luke 22:32; Psalms 42:5, 11; Lamentations 3:26-31 )