John Nelson Darby emphasizes the necessity of grace in producing integrity of heart, as illustrated in Psalm 139. He discusses the psalmist's journey from initial trust in God to a deep desire for self-examination and honesty before Him. Darby warns against the folly of living carelessly, ignoring the reality of judgment, and highlights the importance of confessing sins to experience true relief and integrity. He encourages believers to invite God to search their hearts, recognizing that true integrity comes from understanding God's grace and love, which allows for a genuine relationship with Him. Ultimately, Darby calls for a heart that is open to God's scrutiny, leading to spiritual growth and freedom from guilt.