“Now as to the assurance thou speakest of at the end of thy question. I know in the first place, that though believers themselves do sin, yet they have ‘an advocate with the Father, Jesus Christ the Righteous:’ 1 Jn. ii. 1. And though the doctrine of the gospel be to abstain from all appearance of evil, yet our Lord Jesus Christ is so pitiful, as not altogether to deprive his children of an assurance of their salvation,* though sometimes through weakness they do transgress.
And whereas you would lay an assurance on our obedience to the law; I say, our assurance comes through our believing, and our obedience to the law is a fruit of our believing; for every one that hath this hope, that he is one of the children, or sons of God, by faith in Jesus, 1 Jn. iii. 3. ‘purifieth himself, even as he is pure.’ Holiness of life, if it be right, flows from an assurance of our being justified by Christ’s death on the cross, on Mount Calvary; as it is written again, that he might sanctify his people with his own blood, He suffered without the gate.”
(*For the proof of this, read the good love of God to David, Peter, and others, which did most woefully sin again after they were converted.)
John Bunyan, “A Vindication of Gospel Truths,” The Works of John Bunyan, vol. 2, p. 190.