St. Augustine arguing with Donatists
Question: "I have a recent convert friend who is worried about the validity of his chrismation after the priest left the Church. What should we say to people who received sacraments from priests who either apostatize later or turn out to be grossly immoral, and so now doubt the validity of those sacraments?"
The validity of the sacraments do not depend on the personal worthiness of the priests. This is an issue that was dealt with very early on in Church history. When the dust was settling from the last great Roman persecution of the Church, there was a controversy that arose in North Africa, because there was a bishop, Felix of Aptunga, that was accused of having been a traditor, which means that he was accused of having handed over sacred texts to the Romans to be burned, in order to avoid further persecution. This bishop denied the charge, but the Donatists believed him to be guilty, and therefore declared that any sacraments performed by him were invalid. The Church condemned Donatism as a heresy.
Of course, if a clergyman really is guilty of a sin that warrants he be removed from his office, this should happen, and there are canons that lay out that process. But imagine, for a moment, if we accepted the claims of the Donatists. You could never know for sure if you had received a valid sacrament, because you could never be sure that the priest or bishop performing that sacrament was worthy enough to have performed it. You could never be sure if you were really married sacramentally. You could never know for sure that when you received Communion, that you were really receiving the Body and Blood of Christ. You could never be sure that you were even really baptized. And even if your parish priest was a saint, if the bishop who ordained him was secretly immoral, even his own sanctity would be no guarantee that he was even a real priest.
What the Church teaches is that so long as a clergyman performs sacraments while they are in good standing with the Church, the sacraments he performs are true sacraments. After all, the clergy preside over the sacraments, but they do not perform them alone. They act on behalf of the Church, and the prayers of the entire Church and the grace of the Holy Spirit that is present in the Church ensure that the sacraments are grace-filled.
The teaching of the Church on this subject is not only reassuring to the laity, it is also reassuring to the clergy. I am sure most clergy have a sense of personal unworthiness, and if the validity of the sacraments depended on their personal worthiness, they could not in good conscience attempt to perform them.
It is instructive to consider the prayer that the priest says during the singing of the Cherubic Hymn at the Liturgy:
"None is worthy among them that are bound with carnal lusts and pleasures, to approach or to draw nigh, or to minister unto Thee, O King of glory, for to serve Thee is a great and fearful thing even unto the heavenly hosts themselves. Yet because of Thine ineffable and immeasurable love for mankind, without change or alteration Thou didst become man, and didst become our High Priest, and didst deliver unto us the ministry of this liturgical and bloodless sacrifice, for Thou art the Master of all. Thou alone, O Lord our God, dost rule over those in heaven and those on earth, art borne upon the throne of the Cherubim, art Lord of the Seraphim and King of Israel, Thou alone art holy and restest in the saints. I implore Thee, therefore, Who alone art good and inclined to listen: Look upon me, Thy sinful and unprofitable servant, and purge my soul and heart of a wicked conscience, and, by the power of Thy Holy Spirit, enable me, who am clothed with the grace of the priesthood, to stand before this Thy Holy Table, and to perform the sacred Mystery of Thy holy and immaculate Body and precious Blood. For unto Thee do I draw nigh, bowing my neck, and I pray Thee: Turn not Thy countenance away from me, neither cast me out from among Thy children, but vouchsafe that these gifts be offered unto Thee by me, Thy sinful and unworthy servant: For Thou art He that offereth and is offered, that accepteth and is distributed, O Christ our God, and unto Thee do we send up glory, together with Thine unoriginate Father, and Thy Most holy and good and life-creating Spirit, now and ever and unto the ages of ages."