In his final encyclical, Benedict XVI has this to say about religious freedom:
Religious freedom does not mean religious indifferentism, nor does it imply that all religions are equal. (§55)
Saying such a thing sounds outlandish, maybe, to the modern ear. The 21st century man is accustomed to finding his “truth” from science and nothing else. So religious beliefs are fundamentally irrelevant to his life, and there is no reason to suppose that one religion is true and another false. Benedict rejects this kind of thinking. He knows first of all that science is not necessarily the last word on any subject (given its own history, it is not even the last word on itself!) Secondly, there are truths which are simply inaccessible to reason. The fact that man can’t figure them out does not mean they aren’t true. It means that man has limits. Reason has limits.
Thirdly, precisely because truth cannot contradict truth, it is plainly impossible for all religions to be equal (except of course on the baseless assumption of the materialist who rejects all religions as equally false…but we shall pass that by for now). Example One is trivially obvious: Christians say that Jesus Christ was both God and man; Islam denies this. They cannot both be true, and therefore they cannot possibly be equal.
Hopefully this brief bit of muttering points in a better direction. Benedict says that man must have religious freedom because he cannot be compelled to believe something against his will. Even if an autocrat tried to impose a religion, there is simply no way that he could know whether everyone really believed it because no one knows the human heart except God. So religious freedom is not only necessary by virtue of the fact that we have free will and rational minds, but because no other alternative is possible anyway.
The fact, however, that men cannot and must not be compelled to any particular religious belief does not mean that none of them are true, any more than the fact that differences over anthropogenic global warming means there is no truth about climate change (whatever that truth may be, exactly). As human beings we have a responsibility to pursue the truth and hold to it as best we can. The fact of varying judgments about the content of the truth in any given science (including sacred science) does not mean that no truth exists in them; it simply means that getting at the truth is difficult (something Aristotle said 2300 years ago).