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, and the most focused: its one theme, from the heading, is that she should “willingly drink out her cup of suffering to the end.” God “will not let the needful inflowing of His grace be wanting,” so that she can hold fast “the confidence of faith and the boast of hope in the infinite love of Jesus,” and He will do it Himself, who “has been in all distresses of body and of soul, that He might help us.” Then the central image: “drink, then, fully, the cup which not a foe nor a stranger, but your Father has given you.” Her suffering is not an enemy’s blow but a cup from a Father’s hand, to be received by “offering up yourself and all hardship unceasingly into the fellowship of the sufferings of Jesus.” 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.” The letter closes not with a sentence but a 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 “your 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.