What is the conclusive evidence of the divine origin of known Messengers of God?

What, then, is the mission of the divine Prophets? Their mission is the education and advancement of the world of humanity. They are the real Teachers and Educators, the universal Instructors of mankind. If we wish to discover whether any one of these great Souls or Messengers was in reality a Prophet of God, we must investigate the facts surrounding His life and history, and the first point of our investigation will be the education He bestowed upon mankind. If He has been an Educator, if He has really trained a nation or people, causing it to rise from the lowest depths of ignorance to the highest station of knowledge, then we are sure that He was a Prophet. This is a plain and clear method of procedure, proof that is irrefutable. We do not need to seek after other proofs. We do not need to mention miracles, saying that out of rock water gushed forth, for such miracles and statements may be denied and refused by those who hear them. The deeds of Moses are conclusive evidences of His Prophethood. If a man be fair, unbiased and willing to investigate reality, he will undoubtedly testify to the fact that Moses was, verily, a man of God and a great Personage. (Abdu'l-Baha, The Promulgation of Universal Peace, pp. 363)