This is a highly virtuosic toccata and fugue based on the "Ach Gott und Herr" chorale. It contrasts brilliant, rapid passages with the dense fugal writing.
Requires significant finger dexterity and stamina, especially in the toccata sections.
Ach Gott und Herr (BWV 714) is rated Henle level 6 of 9 — advanced repertoire, roughly ABRSM grades 7–8. In the RCM system it corresponds to about level 9.
You should be comfortable at Henle level 6 — meaning you can already play level 5–6 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
A typical performance of Ach Gott und Herr (BWV 714) lasts about 5 minutes.
Ach Gott und Herr (BWV 714) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.