How can people believe in truths without evidence?
They don’t. What they do is to use previous truths as a proxy for evidence. If executed in a diligent way this is called “the scientific method” — if not, it almost inevitably leads into superstition and prejudice.
This is exactly why we must never stop to question the authority of sources since they act as proxies. We need them as a good trade off between doing the hard work of identifying First Principles for ourselves and moving forward.
Since we usually create a picture of the world based on delivered truths, it pays off putting time, energy or money into ensuring the maximum quality of our sources.