To me, attacking the legitmacy of the institutional Church seems the very definition of Leftism.
Whether Mr. Gibson knows it or not, the Credo that he recites in Latin every week includes a profession of faith in an apostolic Church -- IOW, one that has a visible, hierarchical structure. To be an "apostle" is to be sent, not self-appointed.
True, all true. But none of this should prevent us from sometimes questioning whether there could be subversion from within. I'm thinking about faulty ISL translations of the liturgy, or the gay subculture in the clergy, or flaming liberalism taught in seminaries, or the mal-conceived notion that the Latin right was thrown away at Vatican II (rather than supplemented by the option of the vernacular). If the Vatican does things which seem in error, and which are not backed by declarations of infallibility, what is a Catholic of good conscience to do? Just go along, or ask why, or what?
By calling ourselves Catholic, we are expected to be obedient to our shepherds, no matter how we may feel about the growing liberality of U.S. dioceses. Maybe Mel, et al. need to read scripture and see the importance that God places on obedience over all other things. God asked Abraham to do the unthinkable and sacrifice Isaac (as a foreshadowing of Christ's sacrfice), and Abraham obeyed. God, of course, stopped him. God will surely take care of the Church He built.
Maybe Mel Gibson needs to re-examine his reasons for rejecting the Post-Vatican II Church. Although we may empathize with his love of the Latin rite and pre-V2 trappings, he may eventually find himself more closely identifying with a man named Luther than a man named Peter if he isn't careful...