Not sure how this came up last night.
Ex opere operato: From the work that is done. A term defined by the Council of Trent to describe how the sacraments confer the grace they signify. Trent condemned the following proposition: "That grace is not conferred 'ex opere operato' by the sacraments of the New Law" (Denzinger 1608). Literally the expression means "from the work performed," stating that grace is always conferred by a sacrament, in virtue of the rite performed and not as a mere sign that grace has already been given, or that the sacrament stimulates the faith of the recipient and thus occasions the obtaining of grace, or that what determines the grace is the virtue of either the minister or recipient of a sacrament. Provided no obstacle (obex) is placed in the way, every sacrament properly administered confers the grace intended by the sacrament. In a true sense the sacraments are instrumental causes of grace.
Ex opere operantis: From the work as it is done (by the worker). 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. But it may refer to any subjective factor that at least partially determines the amount of grace obtained by a person who performs some act of piety. Thus in the use of sacramentals or in the gaining of indulgences, the blessings received depend largely on the faith and love of God with which a sacramental is employed or an indulgenced prayer or good work is performed.
