Today I am going to post a comment that Abdu'l-Baha made while in Europe. I think that it lets us know what sort of things he was thinking while he was here in the United States.
Therefore, it must be our task to prove to the thoughtful by reasonable arguments the prophethood of Moses, of Christ and of the other Divine Manifestations. And the proofs and evidences which we give are not based on traditional but on rational arguments.
It has now been proved by rational arguments that the world of existence is in the utmost need of an educator, and that its education must be achieved by divine power. There is no doubt that this holy power is revelation, and that the world must be educated through this power which is above human power.
-- `Abdu'l-Bahá, Some Answered Questions, p. 11