Sacraments are visible, efficacious signs of invisible realities, with the exception of the Eucharist, which is Christ Himself – it is not merely a sign. Sacraments are efficacious, meaning they convey what they signify. They are sources of grace for those who devoutly receive them. Though they in themselves confer grace, the recipient needs to be in a proper disposition (differs with respect to different sacraments) in order to have those graces applied in his life.
We say the sacraments work ex opere operato, meaning they happen “by the very act of their being performed.” Therefore, for a sacrament to happen, the minister of it (person celebrating the sacrament) does not need to be in a State of Grace, leading a holy life, for the sacrament to still work. This is because the power to celebrate the sacrament does not come from Fr. Jones, but from God, and God alone. So, for example, in the Sacrament of Confession, the contrite penitent who confesses his sins to a priest receives forgiveness from God, through the priest. The common Protestant conception of sacraments is that they work ex opere operantis, meaning “by the work of the doer,” meaning the sacrament’s efficacy relies on the holiness of the celebrant. This understanding implicitly misattributes the power to celebrate the sacrament to the celebrant, rather than to God.
Why did God not just perform directly what the sacraments accomplish? The short answer is that we don’t know. We cannot know exactly why. In the end, we must take Him at His Word – that He instituted seven sacraments, dispensed by the Catholic Church for our sanctification. But it also seems fitting that He do so. For example, a contrite penitent who receives absolution after confessing his sins to a priest knows God has forgiven him. If he confessed his sins to God outside of this sacrament, he might be forgiven, but he would not be able to know for sure, unless God gave him a special revelation (which almost never happens). Moreover, the sacraments encourage us to prepare well and give proper thanks to God for His Mercy by providing establishing visible, efficacious signs which we partake in in order to ascend to the Lord, and by enabling certain rites (manners of liturgical practice) to surround the celebration of the sacraments to exalt God by putting His Love, Mercy, Majesty, etc. into perspective for us a bit more.
Every sacrament has three components: matter, form, minister. The matter of the sacrament is the physical sign associated with the sacrament (or that which was at one point a sign). For example, water is the matter of the Sacrament of Baptism. The form of the sacrament is the sacramental rite by which the signs are transformed, either into some qualitatively different substance as in the Eucharist, or into some means of grace beyond that of just a symbol. The form of Baptism are the words: “N., I baptize you in the name of the Father, and of the Son, and of the Holy Spirit.” The minister of the sacrament is the person through whom the Lord performs the sacrament. For example, the minister of the Sacrament of Confession is a priest.
While the sacramental rites have developed through the centuries, the fundamentals of these rites has not, nor has the matter. So while, for example, the Sacrament of Confession, has developed in its form of celebration throughout the Church’s history, its substance has always been the same – namely, that contrite penitents receive forgiveness of their sins from God via the priest.
Sacraments are complete gifts of grace. We do not do anything to “earn” them. Rather, the Lord in His infinite mercy gives these sources of grace to us by His Most Precious Blood, which He shed for us on Calvary.