A technical term literally meaning "from the work of the doer," to be distinguished from ex opere opera-to, which refers to the grace-conferring power inherent in the sacramental rite itself, as an action of Christ. Ex opere operantis refers to the role and value of the recipient's or minister's moral condition in causing or receiving sacramental grace.
Peter of Poitiers (d. 1205) first applied to baptism the distinction between the rite that is performed and the one who performs the rite. Graphically, he compares an action in the natural order to the sacramental action: "When the Jews put Christ to death their deed was evil; but the death of Christ was approved and willed by God" (Sententiarum libri quinque 1, c.16). The application of the principle to the Sacraments was logical, and soon followed. Innocent III (d. 1216) distinguished between sacrilegious action and sacramental celebration: "Although the action of the one who acts (opus operans ) is sometimes unclean, yet always the act done (opus operatum ) is clean" (De Sacro Altaris Mysterio 3.6). In the middle of the 13th century the two formulas were commonly used to point out the difference that exists between Christian Sacraments and Mosaic rites. Actually, the teaching behind the formulas was as old as the doctrine of the objective efficacy of Sacraments, especially of baptism and orders, which Augustine (d. 430) developed against donatism, which asserted that Sacraments administered by notoriously unworthy ministers were invalid.
In the 12th century theologians used the distinction to show that Mosaic rites (with the probable exception of circumcision) conferred grace upon the recipient according only to the measure of his faith and fervor, ex opere operantis; and that, on the contrary, Christian Sacraments confer grace ex opere operato upon the soul capable of receiving it. The Council of Trent (1545–63) defined the term ex opere operato in order to deny the Reformers' contention that Sacraments caused grace exactly as did the Mosaic rites, but it did not deny that the faith and fervor of the (adult) recipient condition the measure of grace received.
Ex opere operantis ecclesiae. Theologians common-ly teach that the only limit to the measure of grace conferred ex opere operato is the degree of faith and fervor in the recipient. This limiting arises, ex opere operantis, from the measure of the recipient's cooperation at the time of receiving the rite.
In the 20th century, theologians began a discussion of the recipient's cooperation, specifically his genuine intention to participate together with the minister in the sacramental action, as a necessary element in perfecting a Sacrament as a practical sign of grace. This discussion enlarged the meaning of ex opere operantis, and is of particular value in determining precisely the active role of the laity in the offering of Holy Mass.
It should be added that the term is not to be confused another technical phrase, ex opere operantis Ecclesiae, that expresses the efficacy of strictly liturgical prayer, an effectiveness that is due to the action of the Church as the Mystical Body of Christ.
Bibliografia: p. l. hanley, The Life of the Mystical Body (Westminster, Md. 1961). c. o'neil, "The Role of the Recipient and Sacramental Signification," Thomist 21 (1958) 257-301, 508-540.