That one must seek no certainty nor comfort outside oneself, but inwardly in God.

What this letter is. A short, deeply tender letter to an anxious sister, dictated when Tersteegen was too weak to write freely — "I keep myself very still… that I scarcely see the paper." Her letter had been read aloud to him by "Brother S.," and his answer is pure comfort wrapped in a mother's affectionate scolding. "But how restless do I see thee, thou silly child!" — the words warm, not harsh — "Canst thou not stay in the lap?" The governing image is God as a nursing mother: "Does not the mother know best… what is needful for thee?" So "let Love do the caring… and do thou only creep straightway again into her lap. She alone is enough for thee." The sister had evidently doubted whether her inward path was right, so Tersteegen redirects her from his words to her own heart: "dost thou then believe only for the sake of my words? Hast thou not thyself… heard the little child in thy heart?" She carries, he insists, "even in the greatest sufferings, an inextinguishable though dark and secret inclination… toward the simple, innocent, gentle, blindly-trusting way of the child" — and, "from time to time… a calming impression of certainty in the Ground, that God would have thee there." That inward witness "can and must be more to thee than the words of men." Then he gives the mechanism precisely: "If thou openest thine eyes by outward-turning, reflection, unrest… then the secret, calming impression… is lost; if thou shuttest thine eyes again, and wilt have no certainty which the Father does not give, but leavest thyself over to Him, then by degrees thou perceivest again the secret calming." The letter closes on its paradox — souls called this way "have nothing when they would have something; but if they let their wanting-to-have fall, then they have everything, even when they seem to have nothing." He signs, simply, "One bound to thee."

A few words and echoes.

• Creep back into the lap. The letter's whole consolation is the image of God as a mother whose child needs only to climb back into her lap. It breathes the air of Isaiah's "as one whom his mother comforteth" (Isaiah 66:13) and "can a woman forget her sucking child?" (Isaiah 49:15). The cure for anxiety is not effort but return.
• Heard the little child in thy heart. His pastoral move is to point her away from his own authority to the inward witness she already carries — "hast thou not thyself… heard the little child in thy heart?" Certainty is not to be argued into her from outside but recognized within.
• The impression in the Ground. His key phrase — "a calming impression of certainty in the Ground" — locates assurance in the deep place of the soul (the Seelengrund), beneath "all thy comprehending and self-making." It is felt, not reasoned; God-given, not manufactured.
• Eyes open, eyes shut. His spiritual physics is exact and practical: turning outward — by "reflection, unrest" — loses the secret peace; shutting the eyes and refusing "certainty which the Father does not give" lets it steal back "by degrees." Assurance is protected by not grasping at it.
• Having nothing, possessing all. The climax is Paul's paradox — "as having nothing, and yet possessing all things" (2 Corinthians 6:10). The anxious soul's very wanting-to-have (a phrase Tersteegen sets off pointedly, rendered here in italics) is what keeps it empty; letting that grasp fall is what makes it rich.