From An Humble Inquiry (Works 12:298):
No, it is not owing to God, nor to any of his revelations, that true saints ever doubt of their state; his revelations are plain and clear, and his rules sufficient for men to determine their own condition by. But, for the most part, it is owing to their own sloth, and giving way to their sinful dispositions. Must God’s institutions and revelations be answerable for all the perplexities men bring on themselves, through their own negligence and unwatchfulness? It is wisely ordered that the saints should escape perplexity in no other way than that of great strictness, diligence, and maintaining the lively, laborious, and self-denying exercises of religion.