If a person claims something and does not provide sufficient evidence, then I would term that person a liar and perhaps a charlatan, and disbelieve all that he/she says further and be wary of him/her. To believe what that person says will be stupid.
It is true that what is sufficient for a person will be different from what another person thinks. That is why Tom, Dick and Harrys are believed. That is a special character of monotheistic religions. In Eastern religions, we have the freedom to dissect it thread-bare. In Western religions it is termed as blasphemy.Why Bahaollah is claiming that there is just one God (other self-proclaimed messengers (prior to Bahaollah and later to him also did that), because then he has to make a claim that he is the messenger of that God (manifestation or whatever, Mirza Ghulam Ahmad was to claim later that he was the mahdi, i.e., returning Jesus). One story feeds the other. No prophet / son / messenger / manifestation / mahdi ever gave a solid proof of existence of any God or their being the sent representatives of God.
That is why these so called prophets / sons / messengers / manifestations / mahdis do not deny the ones who claimed such a status earlier. Because that makes their own story false. Jesus accepted the prophets before him, Mohammad accepted Jesus, Bahaollah accepted Zoroaster, Moses, Jesus, Mohammad and all those who came before him. Later Abdul Baha added the names of Krishna and Buddha too. Every time it is like a new book or episode of Harry Potter or James Bond.