"To defend the truth" means to support and protect what is true or factual. It involves standing up for what is right, trustworthy, and accurate, especially when it is being challenged or questioned. Full definition
Instead of thinking for themselves, they rely on an outside authority instead of the church's authority to defend their truth claims. (religion.blogs.cnn.com)
The rationale for defending truth is that its light shows us the way to freedom. (firstthings.com)
It's a matter, then, of finding the right words and methods to defend the truth of marriage, without producing just another shouting match or a public smear campaign. (faith.org.uk)