The test for the true Church, it seems to me, is not so much the extent to which it excludes evil from itself, but the extent to which it avoids excluding good. All that is good has its ultimate source in God, and so a Church that excludes good is, to that extent, excluding God. We cannot expect the Church to be the sole source of good in the world, for God is larger than the Church even if the Church really is God's true Church. The true Church must therefore be open to the good that has its origin in the world external to it, since such work must finally be a work of God. I don't agree with Catholic apologists who feel they must trace the origin of everything good to Catholicism; it is historically false and philosophically unnecessary. John Paul II, typically, had it right when he acknowledged that the Enlightenment introduced some positive changes into history, among them modern liberal democracy, that have since been acknowledged by the Church. It's no insult to Catholicism that it absorbs good things it did not originate; in fact, the consistency with which it has done this over 2,000 years is powerful evidence of its peculiar and unprecedented origin, for I know of no other institution with a similar record. This goes all way back to the allegedly devastating skeptical point that Christmas is just a pagan holiday taken over by the Church. Yes, Dec. 25 was a pagan holiday, and the Church took it over because pagan holidays are good things, not bad things. The Church takes over what is good in the pagan world and renews it in Jesus Christ, so that it is the same yet reborn. New wine in new wineskins, but the wineskins are still recognizably similar to the old pagan wineskins.
The genius of the Catholic Church, and the reason the gates of Hell will never prevail against it, is that it absorbs whatever is good in its enemies (while, alas, sometimes absorbing their evil as well.) Evil in itself has no power; what is evil has power only the extent that it is good. The enemies of the Church are powerful only to the extent that they are good. The Roman Empire was originally an enemy of the Church, and was also the greatest human institution the world had ever seen. The Church defeated the Empire, not by crushing it, but by transforming it in its spirit, and in the process absorbing the good elements of the Empire - including its language, elements of the old pagan worship, and its hierarchical structure. The Pope is a Roman Emperor transformed in light of the Kingdom of God, and Bishops are Procurators. The Roman Empire is long gone but what was good in it survives in the Church. The Church performs a kind of spiritual jiu-jitsu, taking her enemies' strengths and making them her own, winning the war without apparently fighting.
The intellectual analog of the absorption of pagan worship and political structures was the absorption of pagan philosophy, first that of Plato and later that of Aristotle, and in this century, some Eastern philosophies as well (I am thinking of Thomas Merton and Zen Buddhism.) It takes tremendous confidence and faith for an institution to be open to new philosophies, for there is always the danger that it is the philosophy that will transform the institution rather than vice versa. This seems to be the more or less explicit aim of the many modern philosophical "innovators." Yet the success the Church has seen in absorbing Platonic philosophy and later Aristotelian philosophy is shown not merely in the fact that it remained itself after these exposures, but became more of itself in the process.
I find that the rivals to the Church, religious or secular, disqualify themselves by excluding some aspect of the good. The Mennonites of Pennsylvania, for example, live an admirable life of simplicity, and it is a good life, but it isn't the only good life. The life of the engineer and scientist can also be a good life. So while I admire the Mennonites, I do not think that their religion can constitute the fullness of faith. Secular philosophies that worship science and technology make the same mistake in the opposite direction. The true philosophy will find a way to embrace both alternatives, unifying them in a transcendent truth. G.K. Chesterton:
"It is true that the Church told some men to fight and others not to fight; and it is true that those who fought were like thunderbolts and those who did not fight were like statues. All this simply means that the Church preferred to use its Supermen and to use its Tolstoyans. There must be some good in the life of battle, for so many good men have enjoyed being soldiers. There must be some good in the idea of non-resistance, for so many good men seem to enjoy being Quakers. All that the Church did (so far as that goes) was to prevent either of these good things from ousting the other. They existed side by side. The Tolstoyans, having all the scruples of monks, simple became monks. The Quakers became a club instead of a sect."