The Yin‑Yang represents the Chinese philosophy of how apparently opposing forces interconnect, interact and are mutually dependent in the physical, natural world.
Value judgements, also, have opposites (for example love and hate), and the symbol has been extended to religious philosophy and theology.
Here is an attempt to connect the Yin‑Yang symbol with the Christian cross.
The symbol used by flower-power hippie tribes of the 1960's in their search for things exotic, has now passed on to their sons and daughters. The curved shape's resemblance to ocean waves and the association with nature, makes it a symbol for surfers. It has spread from the beach to snowboard equipment, skateboards, tribal tattoos, and even fantasy computer games designed for 11 year-olds - especially games involving warlocks. (There is a fair sprinkling of Christian Crosses on such items, too.)
The Yin‑Yang symbol originally represented Chinese classical thought of how apparently opposing forces interconnect, interact and are mutually dependent, in the natural world. The philosophy is extended to traditional Chinese science, medicine and martial arts. Male and female, hot and cold, up and down, right and left... our world is full of mutual yet opposing forces which balance. And from that balance, we have life as we know it.
The philosophy has its critics. Yes, there is 'black' and 'white', but there is also 'grey'. There is 'up' and 'down', 'left' and 'right', but there is also 'middle', and 'luke warm' is neither 'hot' nor 'cold'. Gay people do not fit the classic male or female image (me Tarzan, you Jane); but then, very few heterosexuals fit the classic male or female image.
Balance in nature
The natural world is in constant change. A lightning strike causes a forest fire on a mountain (Yang), killing the trees, which in turn leads to soil erosion (Yin), the mountain is destroyed, collapses and nature is in balanced harmony again. But only for a short time, and while that heap of ex-mountain might be resting, other parts of the world see changes taking place. Because nature is dynamic, its balance is always unstable. Balancing, yes. Balanced, no.
And who is to say what 'balance in nature' actually is? We like to think we know and understand nature, but if we're honest with ourselves, we don't really know very much.
All forces in nature have opposites or complements, except one. Gravity. It pulls but doesn't push. Or does it? Quantum field theory says gravity is transmitted by (hypothetical) elementary particles called gravitons. There might well be an opposite particle waiting to be discovered; we just don't know.
When we look at nature, we might think we see balance and we talk audaciously about the 'laws of nature', but we can only see a tiny part of the Universe, so tiny that it's rather insignificant. What we can't see are the destructive forces building up beneath the earth's crust ready to unleash a massive earthquake. We cannot see asteroids hurtling towards earth. We just have no idea what we cannot see.
Balance in mind and spirit
Value judgements, also, have opposites and/or complements (wealth and poverty, happiness and sadness, love and hate, etc.), and so the symbol has been extended to religious philosophy and theology. Some have attempted to connect the Yin‑Yang symbol with the Christian cross.
Balance of Yin-Yang and the Cross
The Yin‑Yang symbol in a Christian context is feeble at best. It could represent the dual nature of Christ (as God and as Human), but that's about the only solid symbolism there is.
Comparing this photo of a necklace with our Yin-Yang Cross image, we can see the orientation of the symbol is not fixed. Conventionally it is seen as the image (right), like sunlight (Yang) cascading down a dark (Yin) mountainside, just as the light of Christ envelopes sinfulness. The Yin‑Yang of nature teaches us that we should strive for balance which will enable us to cultivate the highest and purest development of our spiritual nature.
However, recognising our own spiritual nature (whatever that is) and trying to balance it, is no easier than trying to recognize balance in nature. It doesn't take long to realise that painting our soul with Yin‑Yang philosophy is all a bit dodgy.
Fortunately, the message of the cross couldn't be simpler. The cross means 'love'. And there is nothing simpler, purer, or easier to understand, than love. Love is so pure, that it is purity itself. In fact God, is love. And love is simple, so simple that even a child can use it. Yet love is powerful. Love can destroy evil; evil in all its forms: hatred, envy, jealousy. This is why God is so powerful. (You knew that already.)
Merging the Yin-Yang with the cross implies that with balanced development of our spiritual nature, we can attain love. Most Christians would disagree with that philosophy, because Jesus taught us that good works just aren't enough to open those gates of heaven when we die. The only way to salvation from eternal damnation, is by accepting the love of God.
The Yin-Yang Cross is an attempt to apply a non-religious philosophy to the Christian religion, but it just doesn't fit. Surfer-dragon-tribal crosses are all good fun, but they don't get us to heaven.
That's not an unknown-unknown; we know it for certain.
Q: What's the opposite of gravity?
"As we know, there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns -- the ones we don't know we don't know."
US Secretary of Defense news briefing, February 12, 2002 on the rationale for starting a war www.defenselink.mil/...