On one subject Baha'u'llah remained silent

However, on one subject Baha'u'llah remained silent: designating the person who was to succeed Him. There are many wisdoms in this. Let us return yet again to the analogy of the teacher. It is the teacher's duty to impart knowledge to his pupils and help them in their work, and he is always ready to explain the various subjects to his pupils and answer their questions. But on one occasion he must remain silent and refrain from helping them or answering their questions: namely, on the examination day. On that day the students are left on their own and will have to find the answers by themselves. Only those who pass the examinations are elevated to a higher class, and those who fail are not.

Ref. The Covenant of Baha'u'llah, Adib Tahezadeh