To the same; that she should willingly drink out her cup of suffering to the end.

What this letter is. The third letter to the same sick sister (after Briefs 38 and 39), and the most focused: its one theme, taken straight from the heading, is that she should "willingly drink out her cup of suffering to the end." Tersteegen opens with confidence that God "will not let the needful inflowing of His grace be wanting," so that she can "hold fast, even to the end, the confidence of faith and the boast of hope in the infinite love of Jesus." And He will do it Himself — "He who, for our sakes too, has been in all distresses of body and of soul, that He might help us and have compassion with our weaknesses." Then comes the central image: "drink, then, fully, the cup which not a foe nor a stranger, but thy Father has given thee." Her suffering is not an enemy's blow but a cup from a Father's hand, "according to His unsearchable wisdom and love" — and she is to receive it by "offering up thyself and all hardship unceasingly into the fellowship of the sufferings of Jesus." The second half gives a picture of how faith endures: the body's burdens may "make the spirit faint and cloud it over," but "the childlike faith does not turn aside to these passing little clouds; it suffers on, softly and peaceably," sinking "so much the barer into the love-of-God hidden, unchangeable, in the ground." Even the "clouding-over," he says, works toward salvation — as she will "soon see with our eyes" when set into "an element where no earthly vapors shall rise up any more." The letter closes not with a sentence but with a little couplet of rest: "How good to rest, when toil is past! / How well 'twill do at last!"

A few words and echoes.

• The cup of suffering. The letter's governing figure — a cup to be drunk "to the end" — is Christ's own in Gethsemane, "the cup which my Father hath given me" (John 18:11; Matthew 26:39). The point is whose hand holds it: not "a foe nor a stranger," but "thy Father."
• The fellowship of His sufferings. Her pain is reframed as participation — offering herself "into the fellowship of the sufferings of Jesus" — echoing Paul's desire to know "the fellowship of his sufferings" (Philippians 3:10).
• A high priest who feels our weakness. His reason for confidence — that Christ "has been in all distresses of body and of soul… that He might… have compassion with our weaknesses" — draws on Hebrews' picture of a high priest "touched with the feeling of our infirmities" (Hebrews 4:15).
• Passing little clouds. His image for the moods and burdens of sickness — mere "passing little clouds" the settled faith declines to chase — sets the felt turbulence against the "unchangeable" love of God underneath it.
• Isaiah 60 and the couplet of rest. He points her to Isaiah's promise of a day with "no earthly vapors" — where the Lord is "an everlasting light" and sun and moon are needed no more (Isaiah 60:19–20) — and lets a two-line rhyme carry the hope home: rest after the labor is done.