Excerpts from Books and Wikipedia
"To summarize, not a single soul, whether friend or foe, who attained Bahá’u’lláh’s presence failed to acknowledge and attest to His greatness. Although he might not become a believer, he would invariably bear witness to His greatness. No sooner would someone appear before Him than the encounter would produce such an impression as to prevent him, in most cases, from uttering a word. How often would a bitter enemy resolve in his heart to say such-and-such or to argue so-and-so when he had attained His presence, only to find himself amazed, bewildered, and reduced to utter silence! Bahá’u’lláh never studied Arabic, had a teacher or tutor, or entered a school. Nevertheless His eloquence and fluency in spoken Arabic, as well as in His Arabic Tablets, would astonish the most articulate and accomplished among the Arab men of letters, and all acknowledged that in this His attainments were without peer or equal. . . . For fifty years Bahá’u’lláh withstood His enemies like a mountain: They all sought to annihilate Him; they all assailed Him; they plotted a thousand times to crucify and destroy Him; and throughout those fifty years He was in the greatest peril. . . . In His blessed lifetime Christ educated, in reality, only eleven souls, the greatest of whom, Peter, nonetheless denied Him thrice when put to the test. Notwithstanding this, behold how the Cause of Christ subsequently pervaded the whole earth! In this day Bahá’u’lláh has educated thousands of souls . . .Now, we must be fair and acknowledge what an Educator of mankind this illustrious Being was, what marvellous signs He has manifested, and what power and might have been realized in the world of existence through Him." [Abdu'l-Baha: Some Answered Questions, p. 40-1]