Here Frances Wright answers in the name of Epicurus, as a good summary on several critical questions:
1. What is truth? 2. What is the guide in the search of truth if we discard the will of the gods? 3. Can a false opinion be a crime or a virtue?
From the ending of Chapter 14 of "A Few Days In Athens" (published 1822):
But what is a truth?” said Theon.
“It is pertinently asked. A truth I consider to be an ascertained fact; which truth would be changed into an error, the moment the fact, on which it rested, was disproved.”
“I see, then, no fixed basis for truth.”
“It surely has the most fixed of all — the nature of things. And it is only an imperfect insight into that nature, which occasions all our erroneous conclusions, whether in physics or morals.”
“But where, if we discard the gods, and their will, as engraven on our hearts, are our guides in the search after truth ?”
“Our senses and our faculties as developed in and by the exercise of our senses, are the only guides with which I am acquainted. And I do not see why, even admitting a belief in the gods, and in a superintending providence, the senses should not be viewed as the guides, provided by them, for our direction and instruction. But here is the evil attendant on an ungrounded belief, whatever be its nature. The moment we take one thing for granted, we take other things for granted: we are started in a wrong road, and it is seldom that we can gain the right one, until we have trodden back our steps to the starting place. I know but of one thing that a philosopher should take for granted; and that only because he is forced to it by an irresistible impulse of his nature; and because, without doing so, neither truth nor falsehood could exist for him. He must take for granted the evidence of his senses; in other words, he must believe in the existence of things, as they exist to his senses. I know of no other existence, and can therefore believe in no other: although, reasoning from analogy, I may imagine other existences to be.
This, for instance, I do as respects the gods. I see around me, in the world I inhabit, an infinite variety in the arrangement of matter; — a multitude of sentient beings, possessing different kinds, and varying grades of power and intelligence, — from the worm that crawls in the dust, to the eagle that soars to the sun, and man who marks to the sun its course. It is possible, it is moreover probable, that, in the worlds which I see not, — in the boundless infinitude and eternal duration of matter, beings may exist, of every countless variety, and varying grades of intelligence inferior and superior to our own, until we descend to a minimum, and rise to a maximum, to which the range of our observation affords no parallel, and of which our senses are inadequate to the conception. Thus far, my young friend, I believe in the gods, or in what you will of existences removed from the sphere of my knowledge. That you should believe, with positiveness, in one unseen existence or another, appears to me no crime, although it may appear to me unreasonable: and so, my doubt of the same should appear to you no moral offense, although you might account it erroneous. I fear to fatigue your attention, and will, therefore, dismiss, for the present, these abstruse subjects.”
But we shall both be amply repaid for their discussion, if this truth remain with you — that an opinion, right or wrong, can never constitute a moral offense, nor be in itself a moral obligation. It may be mistaken; it may involve an absurdity, or a contradiction. It is a truth; or it is an error: it can never be a crime or a virtue.”