These are arrangements for solo piano of Bach's liturgical works, likely focusing on movements from his masses. They showcase Bach's contrapuntal mastery and profound harmonic depth, adapted for keyboard performance.
A strong understanding of counterpoint and meticulous articulation are crucial for performing these works accurately.
3 Masses, 235-236 is rated Henle level 5 of 9 — late intermediate repertoire, roughly ABRSM grades 6–7.
You should be comfortable at Henle level 5 — meaning you can already play level 4–5 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
3 Masses, 235-236 is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.