Alright, so first consider the topic of “goodness”. The quality of something being good, and us knowing that it is good. Anything. An apple, a paycheck, an altruistic self sacrifice of life. All these things we know to be good (within reason). Therefore goodness as a quality exists and we can know about it.
In short, this goodness that we notice everyday in a myriad of different things, can be traced back to the singular source of goodness in the universe. The only goodness that there is, and the goodness within all that is good. I call this the highest divine.
Humans aren’t the only ones who recognize what is good for them. All living things do. However, humans are different because of a higher concept of goodness we are able to conceptualize. This goodness is what has allowed us to develop civilization and evolve the way we did.
This goodness was given to us directly from the divine, who was the first one to know what is good and what is not. That being principally, creation and existence. There was a big void of non-existent darkness and it was not-good. So, the highest divine brought forth the universe. Consider the concept “not good” because there are three moral values, not just good and evil. There is good, evil, and not-good (which is also not evil). The void of non-existence being not-good.
Our ability to know what is good came directly from the highest divine as the same power the highest divine possesses to create goodness from nothing. It’s a speck of power, of course, the smallest fragment, but it is the same nonetheless. This is how we know what is good, and when we choose good, we bring more goodness into existence.
Notice, how evil hasn’t even come into the picture yet. All that exists is what is good, and what is not good, with existence being good and non-existence being not-good. Stars exploding, meteors colliding, moons forming, all that being good and not-good. The principal forces of darkness and light that you mentioned in your answer are at play here, darkness also being good when it is in a state of existence, such as nighttime.
That explains generally that we can know what is good, and how it is possible. For specific situations there is an infinitely complex multitude of cultures, societal factors, and circumstances at play at any given moment, so there is a degree of relativity between situations and behaviors, despite there being some general guidelines people universally recognize. Killing another person for instance is widely recognized as evil, but in the defense of one’s home, family, and others it can be good.
It’s this preternatural ability to discern good that allows anybody anywhere to know what is good at any time. Sometimes it can be confusing, and people don’t know because of the complexity of situations and other factors involved, but the ability to know is there. Now, this ability to discern good is the product of sentience. Intelligent thinking and higher thoughts derived from free will, which is a corresponding characteristic of the highest divine from which it came.
Yet, when a human being knows in full awareness what is good and chooses the opposite and goes against that, they engender that sentient energy into non-existent darkness, and evil comes into existence. That is what I call the Evil One. It is a frightening thought, because it is a supernatural sentience of pure evil. However, it is highly imperfect and unstable. It can’t stay in existence for long because existence is good and that causes its entropy which ultimately disperses into elemental darkness of not-good.
However, when a human continues to project their will to do evil, the Evil One stabilizes in reality more substantially and brings about the annihilation and destruction of all that is good, namely all that exists, love being the highest form of existence and what it hates the most. Love is the pinnacle of existence.
So when enough of the Evil One substantiates into reality it can of its own volition influence people to do evil, which they should know better than to do. If this is cause for fear, the surest thing that destroys an Evil One wherever one forms, is elemental light, the purest form coming from the soul of a person, often attributed to what they physically see (but not necessarily).
That’s why evil always hides itself in darkness and deceit, because if people see it, it’s destruction is at hand. If someone watches a crime getting committed, not only is the person likely going to feel guilty, but the witness can take action. This is how corruption in the government should be handled; making their unnecessary secret doings public and reveal them to the light of day. If they are good, they should have nothing to fear.
So determining good in any situation requires reason and wisdom. You can explain good. Evil can’t explain itself because it has no right to exist. This is the beauty of argument and free speech, and why I am so upset by all the censorship that happens in public discussion in the USA.
That’s about all I need to say to answer that, but I want to tell you something else, because it is directly related: The meaning of life.
Why do we have this ability to know what is good and create good with our judgment?
Consider mathematical infinity. It is an ever increasing number. It can never be static. It has to always be in motion of increasing and decreasing value. The highest divine is infinite. However, when I write that, it implies stasis. “God is infinite” is a static concept, contained in the word “infinity”.
How can the highest divine continue to be truly infinite then if it is expanding in all directions through all time? By engendering the ability to create more good in living beings separate from its true singular form. Therefore, when a human being recognizes what is good, chooses it, and brings it into existence, the goodness of God expands in a way that is interdimensional through consciousness. So, the meaning of life is basically to continue expanding goodness beyond conventional reality through the independent free will of a multitude of individual beings.
Yet why does the highest divine care? This is where the Story comes in. The highest divine cares because it is a good story at the end of time and creation. A good story from which wisdom is derived. Wisdom that the highest divine uses to recreate existence all over again in a new and better way.
I actually learned most of this by writing an experimental science fiction novel based on my intuitive perception of reality and faith in divine love. Afterwords, I spent years studying, learning, and thinking it through along with my life experiences, until eventually I arrived at solid conclusions like I just explained.
I hope this is of use to you, and also that it is enjoyable to read and contemplate, because this is just the fundamentals. There is an infinite amount of progress to be had by delving further.