Mankind is flawed, and all the thinking and humanism shows us is that mankind, when left to our own devices, are flawed and failed individuals.
It's like the argument in The Patriot. Why should I trade a tyrant one thousand of miles away (The Vatican/Pope/Church) for 1,000 tyrants one mile away?
The list of things the Church has been wrong about, is lengthy. But if we use the failures of people and institutions as a benchmark to disqualify them from demonstrating moral authority, then the individualists, humanists, and everyone else who is thinking and developing their own moral code, are just as flawed of individuals, just as hypocritical as the Church, etc.
It's fine, the Church isn't perfect, no one has ever argued it was. But if perfection is required for moral authority, we're pretty well fooked, right?
I see no reason to think individuals are better equipped or able to discern moral authority through thought and contemplation.