catechism n.

"oral instruction, catechism," 1753, from Latinized form of Greek katēkhesis "instruction by word of mouth," from katēkhein "to instruct orally," originally "to resound" (with sense evolution via "to sound (something) in someone's ear" to "to teach by word of mouth"). This is from kata "down" (in this case, "thoroughly;" see cata-) + ēkhein "to sound, ring," from ēkhē "sound" (see echo (n.)).