A very good answer, Khalid. But one question, if I may - does the effectiveness of a priest’s absolution depend upon our subjective state? Forgive me if I misinterpret your answer to suggest so.
Yes, it does. The priest can issue the formula of absolution, but if the man being absolved is contrite in neither of the ways mentioned above, for example, in a man who intends to sin again (not merely the knowledge that he likely will, but the intent to), or for a man who feels no sorrow for his sin nor fear of its repercussions, the absolution is not efficacious (although, technically, still valid: it just has no power to forgive sins in an unrepentant individual).
The subjective state of the
priest has nothing to do with the validity of absolution - the priest can be an unrepentant, non-contrite, habitual mortal sinner who revels in his sin, and as long as he speaks the words, and the penitent is in a state of either imperfect or perfect contrition, the absolution is valid and efficacious.
Ex opere operato (out of the act acted, or out of the performance performed) relates to the efficacy and validity of sacraments performed by an unworthy priest (the sacraments are valid), not upon the efficacy of sacraments
in general. Reception of grace through sacraments is not automatic: the recipient must be properly disposed, which is to say, at least open to the possibility of receiving grace through the sacrament.
One can analogize, to a point, with Holy Communion (an unrepentant mortal sinner receiving communion commits another mortal sin), but the analogy is flawed and limited, as the Real Presence is still there, no matter the state of the recipient.
A better analogy may be to marriage, and the canons that allow for its annulment (except marriage is a sacrament granted by husband to wife and vice versa, not by the Priest, in Latin theology; Orthodox and Eastern believe that the Priest performs the sacrament of matrimony). If a man presents to be married, but has no intent to remain faithful, or is incapable of consenting, etc. (according to the relevant canons), and goes through all of the relevant rituals, the sacrament never was performed, and the inviolable marriage bond never formed: the form was correct but the matter not.
However, all of these analogies are inadequate and break down if pressed too far.