Composed for the Twenty first Sunday after Trinity, which fell on October 17, 1723, Bach's Cantata No. 109 "Ich glaube, lieder Herr, hilf meiem Unglauben" (I believe, dear Lord, help my unbelief) (BWV 109) sets a text by an anonymous writer with its first movement taken from the Gospel According to St. Mark 9:24 and its chorale finale from Lazarus Spengler. The cantata is scored for alto and tenor soloists, chorus, a pair of oboes, strings, and basso continuo and a corne du chasse in the outer movements. "Ich glaube, lieder Herr, hilf meiem Unglauben" is in a succession of rising tonalities starting its first movement in D minor, its second starting in B flat major and ending in E minor, its third in E minor with a central section in A minor, its fourth starting in G major and ending in D minor, its fifth in F major, and its sixth starting in D minor and ending in A minor. The first movement is a massive chorale fantasia in da capo form for full orchestra, including an independent corne du chasse. The second is a dynamically shaded secco recitative for tenor soloist and continuo that blossoms into a melismatic Adagio in the closing bars. The third is a da capo aria for nimble tenor soloist, virtuostic first violins, strings, and continuo. The fourth is a very brief secco recitative for soloist and continuo. The fifth is a dancing triple-time da capo aria in the form of a trio sonata for alto soloist, a pair of oboes, and continuo. "Ich glaube, lieder Herr, hilf meiem Unglauben" concludes with a second huge chorale fantasia movement with the chorus intoning Spengler's hymn against an independent instrumental ritornello.

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