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 theologians today are very ignorant of the balance struck between ex opere operato (literally, from the work performed) and ex opere operantis (literally, from the work of the doer) as elucidated by the Holy [...]