Johann Sebastian Bach composed the church cantata O Ewigkeit, du Donnerwort (O eternity, you word of thunder),^[1] BWV 20, in Leipzig for the first Sunday after Trinity and first performed it on 11 June 1724. Bach composed it when beginning his second year as Thomaskantor in Leipzig. It is the first cantata he composed for his second annual cycle which was planned to contain chorale cantatas, each based on a Lutheran hymn. The cantata is focused on Johann Rist's 1642 hymn "O Ewigkeit, du Donnerwort", with a chorale melody by Johann Schop. As usual for Bach's chorale cantatas to come in the cycle, selected hymn stanzas were retained while the others were paraphrased by a contemporary poet who transformed their ideas into a sequence of alternating recitatives and arias. For this cantata, the first stanza was used unchanged, and two more stanzas to conclude the cantata's two parts. The first part was performed before the sermon, the second part after the sermon. The first part is in seven movements, and the second part is in four movements.

Bach scored the cantata for three vocal soloists, a four-part choir and a Baroque instrumental ensemble of two tromba da tirarsi, three oboes, two violins, viola and basso continuo. Schop's chorale melody appears in the opening chorale fantasia and in the identical four-part harmonisation closing the cantata's two parts. All instruments play in the opening chorale fantasia, in which the soprano sings the hymn tune as a cantus firmus. It is written in the style of a solemn French Overture, opening both the cantata and the second cantata cycle.

Bach composed the cantata in 1724 for the First Sunday after Trinity.^[2] The Sunday marks the beginning of the second half of the liturgical year, "in which core issues of faith and doctrine are explored".^[3] The year before, Bach had taken office as Thomaskantor in Leipzig. He was responsible for the education of the Thomanerchor, performances in the regular services in the Thomaskirche, the Nikolaikirche and others. During his first year, he started composing one cantata for each Sunday and holiday of the liturgical year,^[4] termed by the Bach scholar Christoph Wolff as "an artistic undertaking on the largest scale".^[5] In 1724, Bach began exclusively composing chorale cantatas for his second annual cantata cycle, beginning with this cantata and totaling some 40 chorale cantatas by the end of the cycle. Each cantata was based on the main Lutheran hymn for the respective occasion.^[6] Leipzig had a tradition of focusing on the hymns. In 1690, the minister of the Thomaskirche, Johann Benedikt Carpzov, announced that he would preach also on songs and that Johann Schelle, then the director of music, would play the song before the sermon.^[7]

The prescribed readings for the Sunday were from the First Epistle of John, "God is Love" (1 John 4:16–21), and from the Gospel of Luke, the parable of the Rich man and Lazarus (Luke 16:19–31). The text is based on Johann Rist's hymn "O Ewigkeit, du Donnerwort", which was published in the collection Himlische Lieder (Heavenly songs) in Lüneburg in 1642.^[8] The text is based on 12 of the hymn's 16 stanzas.^[3] The hymn, reflecting death and eternity, corresponds well to the parable of the rich man who has to face death and hell.^[7] It is subtitled "Ernstliche Betrachtung der unendlichen Ewigkeit" (A serious consideration of endless eternity).^[9][10] The text of three stanzas (stanzas 1, 8 and 12, used for movements 1, 7 and 11) is kept unchanged.^[11] An unknown author rephrased the other stanzas of the chorale to recitatives and arias, generally alternating and using one stanza for one cantata movement. The poet combined two stanzas, 4 and 5, to form movement 4. He used the lines "Vielleicht ist dies der letzte Tag, kein Mensch weiß, wenn er sterben mag" (Perhaps this is your last day, no one knows when he might die)^[1] from stanza 9 in movement 9 which is otherwise based on stanza 10. In movement 10, he inserted a hint at the Gospel. Overall, the poet stayed close to the hymn's text, which is characteristic for the early cantatas in Bach's second annual cycle.^[11] The poet was possibly Andreas Stübel, who died in 1725, which would explain why Bach did not complete the full cycle, but ended on Palm Sunday.^[7]

The chorale theme was composed by Johann Schop for the hymn "Wach auf, mein Geist, erhebe dich", which appeared in the collection Himlische Lieder. It was assigned to the text by Johann Franck in his 1653 edition of Praxis pietatis melica. The tune is featured in all three movements which use Rist's original text.^[12]

Bach first performed the cantata on 11 June 1724.^[13]

The entries of the following table are taken from the list of recordings provided on the Bach Cantatas Website.^[19] Ensembles playing on period instruments in historically informed performance and a choir of one voice per part (OVPP) are marked by green background.