Baha’u’llah addresses the whole of Christendom

“Followers of the Gospel,” Bahá’u’lláh addressing the whole of Christendom exclaims, “behold the gates of heaven are flung open. He that had ascended unto it is now come. Give ear to His voice calling aloud over land and sea, announcing to all mankind the advent of this Revelation—a Revelation through the agency of which the Tongue of Grandeur is now proclaiming: ‘Lo, the sacred Pledge hath been fulfilled, for He, the Promised One, is come!’” “The voice of the Son of Man is calling aloud from the sacred vale: ‘Here am I, here am I, O God my God!’ … whilst from the Burning Bush breaketh forth the cry: ‘Lo, the Desire of the world is made manifest in His transcendent glory!’ The Father hath come. That which ye were promised in the Kingdom of God is fulfilled. This is the Word which the Son veiled when He said to those around Him that at that time they could not bear it… Verily the Spirit of Truth is come to guide you unto all truth… He is the One Who glorified the Son and exalted His Cause…” “The Comforter Whose advent all the scriptures have promised is now come that He may reveal unto you all knowledge and wisdom. Seek Him over the entire surface of the earth, haply ye may find Him.” 

(‘The Dispensation of Baha’u’llah; included in ‘The World Order of Baha’u’llah’)