God reveals Himself in the person of His Manifestation

Every time God the True One—exalted be His glory—revealed Himself in the person of His Manifestation, He came unto men with the standard of “He doeth what He willeth, and ordaineth what He pleaseth.” None hath the right to ask why or wherefore, and he that doth so, hath indeed turned aside from God, the Lord of Lords.

-Baha'u'llah

http://reference.bahai.org/en/t/b/ESW/esw-4.html