In "Self-Reliance," what is the only law Emerson says can be sacred to him?
Emerson believed in two kinds of "learning" --- tuition and intuition. Tuition is the kind of learning we generally associate with school. It comes from effort, from absorbing knowledge from those around us. The other is intuition, a direct perception that each of us has to the truth because we all participate in one "whole" --- the "Oversoul." This is a basic principle of Transcendentalism ... that genius comes not so much from the accumulation of facts, but from direct inspiration from this Oversoul.
Because we have a direct contact to this knowledge, Emerson and Transcendentalists believe that these perceptions are the only "Law" to which we are responsible. This will not go over so well with the "most men" who "live lives of quiet desperation" (Thoreau). For trusting these perceptions, the "world will whip you with its displeasure." Whereas most men want us to share the worldview of the majority, Emerson asserts that the only time we are ourselves is when we "Trust Thyself" as did Moses and all other truly great men.
"Trust thyself; every heart vibrates to that iron string."
"Whoso would be a man must be a non-conformist."
Here is the quotation from "Self-Reliance":
Whoso would be a man must be a nonconformist... Nothing is at last sacred but the integrity of your own mind....I remember an answer which when quite young I was prompted to make to a valued adviser, who was wont to importune me with the dear old doctrines of the church. On my saying, What have I to do with the sacredness of traditions, if I live wholly from within? my friend suggested, — "But these impulses may be from below, not from above." I replied, "They do not seem to me to be such; but if I am the Devil's child, I will live then from the Devil." No law can be sacred to me but that of my nature. Good and bad are but names very readily transferable to that or this; the only right is what is after my constitution, the only wrong what is against it. A man is to carry himself in the presence of all opposition, as if every thing were titular and ephemeral but he. I am ashamed to think how easily we capitulate to badges and names, to large societies and dead institutions. Every decent and well-spoken individual affects and sways me more than is right. (boldface added)
Rather than look to a supreme being to supply laws for what is right and wrong, Emerson says that one's own good sense--"the integrity of your own mind"--is what each person should rely on.