Bab and Bahá did not Reveal everything, so another Prophet will come soon after them

Bah'u'llh, as it was recorded in the writings of Nabil, Taherzadeh, and Shoghi Effendi, was constantly receiving revelation from God in which He had to destroy . . . the Plan of God, was not yet finished. In the writings of Shoghi Effendi we find confirmation of this:

"As to those verses which He either dictated or wrote Himself, their number was no less remarkable than either the wealth of material they contained, or the diversity of subjects to which they referred. A vast, and indeed the greater proportion of these Writings were, alas, lost irretrievably to posterity. No less an authority than Mirza Aqa Jan, Bah'u'llh's amanuensis, affirms, as reported by Nabil, that by the express order of Bah'u'llh, hundreds of thousands of verses, mostly written by His own hand, were obliterated and cast into the river. 'Finding me reluctant to execute His orders,' Mirza Aqa Jan has related to Nabil, 'Bah'u'llh would reassure me saying: 'None is to be found at this time worthy to hear these melodies.' Not once, or twice, but innumerable times, was I commanded to repeat this act.'" (God Passes By, 138)