Baha'u'llah has shown that a Messenger from God can also be called a Manifestation of God. Thus the Bab, Muhammad and Christ, to name a few, were Messengers/Manifestations of God. The Essence of God can not be known, but all we can know of God is what the Manifestations show us and tell us about.
All these Manifestations share the same Essence;
"Know thou assuredly that the essence of all the Prophets of God is one and the same..." -- Bahá'u'lláh
Man can never have access to what is God, the door is closed, Gods mercy allows us to have a glimpse;
"The door of the knowledge of the Ancient Being hath ever been, and will continue for ever to be, closed in the face of men. No man's understanding shall ever gain access unto His holy court. As a token of His mercy, however, and as a proof of His loving-kindness, He hath manifested unto men the Day Stars of His divine guidance, the Symbols of His divine unity, and hath ordained the knowledge of these sanctified Beings to be identical with the knowledge of His own Self.... Every one of them is the Way of God that connecteth this world with the realms above..." Baha'u'llah
Thus to accept the Manifestation is to accept the knowledge of God, all we can know of God.
Regards Tony