Fr. John Hardon SJ is quoted at The Fatima Center defining these two sacramental terms:   Ex opere operantis is a term mainly applied to the good dispositions with which a sacrament is received, to distinguish it from the ex opere operato which is the built-in efficacy of a sacrament properly conferred.   The above is an excellent definition of those two sacramental terms that always must be considered in a balance.  Most amateur theologians today are pretty good at knowing the principle of ex opere operato, namely, that a priest in mortal sin still validly confects the sacraments (provided he say the correct words.)  This is good that amateur theologians know this.  But most amateur [...]