Not To Be Confused With
Catholics and Orthodox count seven sacraments (or “mysteries”); most Protestants count two (baptism and the Supper) as instituted by Christ. “Sacrament” (Latin) and “mystery” (Greek, mystērion) name the same reality.
Augustine's phrase “a visible sign of an invisible grace” is the classic shorthand. Traditions divide over number, and over how the sign relates to the grace — from effecting it (ex opere operato) to sealing and signifying it. The Lord's Supper and baptism carry most of the debate.