Your institution is morally bankrupt.
The objection, in full
The Catholic Church spent decades concealing the systematic sexual abuse of children by its clergy. The Borgia popes ran the Vatican as a criminal enterprise. The Vatican Bank has been implicated in money laundering across multiple decades. Inquisitors burned dissidents. Bishops blessed colonial conquest. An institution with this record is not a credible vehicle of divine truth, regardless of what its doctrines claim on paper.
The Church has been weighed in the balance and found wanting.
or, in plain terms —If the Church were really God's, why is it so corrupt?
The Donatists asked this question first. The Church gave its answer.
In my own day there was a movement, the Donatists, who held that the sacraments administered by sinful or compromised clergy were invalid. The bishops who had handed over the Scriptures during the persecutions, they said, had forfeited their priesthood; the Church needed to be a Church of the pure. I argued against them, at length and with the support of councils, and the Church's settled answer is this: the validity of the sacrament does not depend on the holiness of the minister, but on the action of Christ working through him.
This is not a convenient excuse. It is a hard-won doctrine, paid for by a long controversy. If the worthiness of the minister determined the validity of the sacrament, no one could ever be sure his baptism was real, his marriage sacramental, his absolution effective — because no one can be sure of another's worthiness, and most of the time the minister himself cannot be sure of his own. The doctrine ex opere operato — that the sacrament works by the work worked, not by the worker — is what makes the sacramental life possible at all.
The corollary is the one the objection misses: the Church has always known she was full of sinners, including in her sanctuary. The Catholic position is not "look how holy we are." It is "look how holy the grace is that works even through us." The historical record of clerical corruption is not a refutation of this teaching. It is the situation the teaching was developed to address.