If someone only claims to believe something is true, but not to say that it is true, then they don’t make any truth claim at all. And, therefore, cannot be used to argue for the relativity of truth.
If I simply say that I believe this, then that is relative. But if I say it is true, then I’m making an objective truth claim and my statement can be proven to be either true or false.
But if I say “I believe,” you can’t prove or disprove that; I am simply just declaring my beliefs. So belief statements are not truth statements! Truth is a justified belief, or a justifiable belief.
— Norman Geisler