There are two major religions on Gethen. Karhide has Handdara while Orogreyn has Yomeshta. Each roughly corresponds to a major Earth religion.
Handdara represents a Tao or Zen type religion. It takes much of its inspiration from Lao Tzu's Tao Te Ching. Here are some examples of what we're talking about:
From the Tao:
When you wish to contract something,
you must momentarily expand it;
When you wish to weaken something,
You must momentarily strengthen it. (80.1)
And here's Estraven:
To be sure, if you turn your back on Mishnory and walk away from it, you are still on the Mishnory road. To oppose vulgarity is inevitably to be vulgar. You must go somewhere else. (11.19)
Once more from the Tao:
The bright Way seems dim.
The forward Way seems backward.
The level Way seems bumpy.
Superior integrity seems like a valley. (3.2)
And Estraven brining it home:
Light is the left hand of darkness
And darkness the right hand of light.
Two are one, life and death, lying
Together like lovers in kemmer. (16.44)
Taoism is the Chinese philosophy that gave us the yin yang, and similarly, Handarra places its focus on the balance and unity of opposites. Instead of seeing light as good and darkness as evil, Handarra sees both as part of a necessary balance.
Other opposites that Handarra considers important to keep in balance are unlearning in order to learn (5.90), inaction as the best possible action (5.113), and seeking knowledge by not seeking answers (5.107).
And while our heads are still hurting with all these opposites, you can check out more in our Shadows and Androgyny entries in "Symbols, Imagery, Allegory" section. Go on, it'll be fun.
The Yomeshta more closely follows Western religions, particularly the Judeo-Christian tradition. It originates with an all-knowing prophet named Meshe. He preaches against darkness, drawing his knowledge from the light instead.
The Yomeshta consider the Handarra, "those that call upon the darkness," wrong, for "there is neither darkness nor death, for all things are, in the light of the Moment" (12.12-13). That tone will sound pretty familiar to anyone who has ever been to Vacation Bible School.
Keeping with the idea of unity from opposites, both the Yomeshta and the Handarra religions come from similar origins and are trying to solve similar problems. Meshe was a Handarra foreteller before he was asked to answer the question, "What is the meaning of life?" (5.44). And each religion is trying to contemplate the nature of shadow and light in relation to knowledge.
Perhaps a small commentary on the religions of our own world, hmm?