Richard Sibbes on assurance:
“God can see somewhat of his own Spirit in that confusion, but the spirit [of the believer] itself cannot. Then go to the blood of Christ! There is always comfort. … Go … to the blood of Christ, that is, if we find sin upon our consciences, if we find not peace in our consciences, not sanctification in our hearts, go to the blood of Christ, which is shed for all those that confess their sins, and rely on him for pardon, … before we go to Christ it is sufficient that we see nothing in ourselves; no qualification; for the graces of the Spirit are not the condition of coming to Christ, but the promise of those who receive Christ after. Therefore go to Christ when thou feelest neither joy of the Spirit, nor sanctification of the Spirit; go to the blood of Christ, and that will purge thee, and wash thee from all thy sins.'”
Citation: Richard Sibbes, Learned Commentary … upon the First Chapter of the Second Epistle … to the Corinthians (Works, vol 3, p 464), quoted in The Westminster Directory of Public Worship, Discussed by Mark Dever and Sinclair Ferguson. (Christian Heritage Publishers, 2008; p 52).