Duality, for me, in the Catholic sense meant that there is "God" and then there is little old me. Non-duality is the realization that is available to all humans that we are not actually separate from the Tao, God, the Universe, or whatever label you care to place on the invisible power that governs the realm that we inhabit. You are an interconnected piece of life that through consciousness can impact future frames of life that are generated by the infinite universal computer that plays the movie in our mind. (Yes, you can come to have direct experience and knowledge that this power exists but it is a super-rational process that most of our left-brained peers, in our masculine, logic dominated society dismiss out of hand. This realization has been called Enlightenment, Awakening, Waking Up, etc)
Back to duality. We must remember that light and darkness are not opposites, they are the ends of a spectrum measuring the same thing: amount of light. Our western mindset has these placed as opposites, but they are not, they are the same thing. This is the principle of polarity in Hermeticism. The main insight is that these things are not different, they are the same thing in varying degrees (number of photons, in this case). Here is a decent overview (Look for Principle of Polarity, although the #1 point that the universe is a giant mind is the most important to me) https://medium.com/the-mission/the-hermetic-revival-7-ancien...
I'd highly recommend reading The Hermetica. It's not exactly an ancient source text, but it attempts to distill the wisdom of the body of knowledge called Hermeticism.
Your research is going to take you into works described as "occult". This bothered me at first, but then I realized it was my Catholic brainwashing about how occult is somehow related to satan or something. Occult simply means "hidden". These occult teachings, which every spiritual tradition possesses, are the pearls that were not to be cast before swine. This knowledge was typically reserved for initiates into a particular tradition. Basically, this means you had to be really committed and show your worth before you were taught this stuff. This is what many of the historical secret societies were protecting and teaching. This is what you learn when you reach the highest degrees of Freemasonry. Thankfully, the internet is blowing the lid off of this occulted information and people are waking up to the nature of reality at a great rate!
You are on a nondual Christian track reading the Gospel of Thomas. This Quora post sums it up nicely:
https://www.quora.com/What-are-the-teachings-of-nondual-Gnos...
Here are some ideas of what life looks like post realization of your nondual nature. #6 describes it well:
https://www.consciouslifestylemag.com/spiritual-awakening-si...
Best to you on your Path!