From the Latin, meaning "by the work of the doer." In sacramental theology, the phrase concerns the proper disposition of the minister or recipient of a sacrament. It does not deny the objective reality of the sacraments, but it indicates that the proper disposition is needed for the sacrament's full effectiveness in the life of the participant. For example, lack of faith can block or hinder the effectiveness of a sacrament in a person's life.
Ex Opere Operantis
Glossary definitions provided courtesy of Church Publishing Incorporated, New York, NY,(All Rights reserved) from "An Episcopal Dictionary of the Church, A User Friendly Reference for Episcopalians," Don S. Armentrout and Robert Boak Slocum, editors.