For starters, sacramentals only have potential for giving us grace and that we must be properly disposed to receive such graces from God.
Sacramentals are not sacraments. Sacraments were instituted directly by Christ Himself, while sacramentals are instituted by the Church. This means that they do not have the same type of saving grace as the seven sacraments. Instead, they convey grace through the “work and prayers of the Church” (ex opere operantis Ecclesiae) and “by the work of the doer” ex opere operantis. This means that the grace put forth by God is more dependent on the disposition of the person performing the act.
This is different from the performance of the seven sacraments. Christ Himself supplies what is lacking in the priest who celebrates the sacraments, so that grace is able to be communicated no matter what. If a priest is sinful and says Mass, the Eucharist will still be transformed into Christ’s body and blood (provided that the priest uses the proper words and matter required for the sacrament). The faithful still need to have a proper disposition to receive all the graces of God in a sacrament such as the Eucharist, but God’s action is much more powerful in a sacrament than any sacramental.
The last important distinction is that Christ instituted the seven sacraments and the Church establishes sacramentals. The Church does so under the inspiration of the Holy Spirit, but because they were not founded directly by Christ, sacramentals can ebb and flow according to the circumstances and place. Even the local Bishop, with proper approval from the Pope, can establish a sacramental for use in his diocese. Sacramentals can even be disbanded if they no longer serve a purpose (for example, the blessing of a telegraph is one that I doubt gets used anymore).
So as you can see, sacramentals, while related to the seven sacraments, do not operate in the same way. This affects how we use them and teaches us to be properly disposed to the graces God wants to give to us through the sacramentals.
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