The full text of “Self-Reliance” does not contain the word “sacred” in the opening paragraphs. Perhaps you are reading an abridged version. The word does appear twice in the 7th paragraph, as follows:

Whoso would be a man, must be a nonconformist. . . Nothing is at last sacred but the integrity of your own mind. . . 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.

In both instances, “sacred” does not refer to any tenet of an organized religion or a spiritual faith; rather, it is used to describe the importance of having sure confidence and faith in oneself. If you consider your personal integrity and individual nature to be sacred, you respect them and hold them in as high esteem as a devoted follower would hold hallowed ground. You believe wholeheartedly in yourself, your subconscious, and your own higher law. You protect your core from the outside influences or social expectations of others. “Self-Wisdom” is one of the major themes of Transcendentalism (along with the Importance of Nature and Commitment to Social Reform). Being able to rely on oneself and know and adhere to one’s beliefs are among the key foundations to living a transcendental life.