Muir's father was deeply religious and John eventually memorized most of the bible. This upbringing is reflected in the 1856 draft letter to a friend in his Religious Essays; Log School etc Notebook. Later, Muir frequently filled his writings with religious words and phrases.
"O dear friend let us give our hearts to Christ our Savior and love him and follow in his footsteps forever. Then however far we each follow our destiny here, we shall meet again above the region of storms in that bright mansion of the blessed home of our Saviour & Father to part again no more forever." (John Muir Papers Religious Essays; Log School etc., 1856, 1860 [ca. 1887], Image 7)
For more information...
Read The Young John Muir: An Environmental Biography, 1999 by Steven J. Holmes.