We have no real value just because value isn't real, but a property of knowledge.
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While things themselves aren't good or evil, facts serve as the basis of our knowledge of them as good or evil. Inherent significance is an oxymoron, there is only extrinsic meaning,
For most this line of reasoning takes us to utter materialism. Meaning becomes not only extrinsic but also relative. What is factually and demonstrably desirable for me becomes the definition of good. If I have the power to enforce my will I should always do so provided I can avoid negative repercussions. Any harm inflicted upon others is meaningless for those others have no value beyond their usefulness to me. This worldview leads to bondage, suffering and stagnation.
Alternatively, the knowledge of an infinite Creator who formed existence out of nothing and maintains creation leads us to the derivation of something not only functional but also wonderful and elegant. This is the knowledge that allows man to escape from bondage and transform himself into something better.
Of course the rationalists in history were all theists, and monotheists... yet they were wrong...
every star and stone and every mountain slope is a proof of impossibility of God's existence
Your argument that the vastness of time and creation somehow disprove God are unconvincing. These things simply give us a small taste of what infinite truly means.
And how to be certain in the knowledge of (the) idea of good? You have to deduce it a priori, much like a mathematical proof.
My a priori deduction of good is likely to differ from yours nihilnegativum. My definition of good is that of an infinite Creator. Others will define it as some physical pleasure and fall into the materialism described above. You have conspicuously failed to provide us with your definition.