Here's what some non-Muslims have to say about Prophet Muhammad Sallallaho Wasallam, not that Muslims need approval from non-Muslims to revere our Prophet (saw)! But it is an interesting read nonethless.

• Michael H. Hart (an American Historian, Researcher and Mathematician) in his published book, entitled “The 100: A Ranking of the Most Influential Persons in History”, puts Muhammad (saw) on top of the list — even more influential than Jesus Christ who is only third in rank. Hart gives the following brief but very concise reason why he chose Muhammad (saw) as the “Number One” most influential man in history:

  “... He was the only man in history who was supremely successful on both the religious and secular levels. Of humble origins, Muhammad founded and promulgated one of the world’s great religions, and became an immensely effective political leader. Today, thirteen centuries after his death, his influence is still powerful and pervasive.”
  (Hart, 1978 p. 33)

• “Head of state as well as of the church, he was Caesar and Pope in one; but he was Pope without the Pope’s pretensions, Caesar without the legions of Caesar. Without a standing army, without a bodyguard, without a palace, without a fixed revenue, if ever any man had the right to say that he ruled by the Right Divine, it was Muhammad, for he had all the power without its instruments and without its supports.”
  (Borsworth Smith , 1874, cited by Rahman, 1981, p. 183)

• “Philosopher, orator, apostle, legislator, warrior, conqueror of ideas, restorer of rational dogmas, of a cult without images; the founder of twenty terrestrial empires and of one spiritual empire, that is Muhammad. As regards all standards by which human greatness may be measured, we may well ask, is there any man greater than he?”
  (Lamartine, Vol. II, 1854, p. 277. Quoted in WAMY Series in Islām No. 4, “What They Say About Muhammad?”)

• “Muhmmad’s(s.a.w) unique position in the religious history is due to the fact that he inspired all he did without being a saint or an angel, without having any attributes which were not strictly human. Outside his tremendous personality, he had nothing in life to distinguish himself from the other Muslims.
  (The messenger by R.V.C. Bodley)

• “Judged by smallness of the means at his disposal and the extent and permanence of the work he accomplished , no name in the worlds history shines with amore specious lustre than that of the Prophet of Mecca”
  (Islam and it’s founder by W.H.Stobart)

• “it’s a vulgar opinion that ‘Mahomet propagated his Doctrine by the sword; and not only compelled the Arabians at the first to receive his Religion but obliged his successors by a perpetual vow.But how generally so ever this be believed and how great so ever they who support it, yet it is no other than a palpable mistake.It is very true that Mahomet did levy wars in Arabia but it was with the object of restoring an old Religion,not to introduce a new one.He taught his followers to abolish idolatry everywhere and that all the world was obliged to the profession of his Religion or that He compelled any therets,is a falsehood.
  (D.S Margoliouth)

• “But in the case of Islam,there can be no mistake.True,the Arabs in themselves were a great and virile people ,But it was the genius of Muhammad , the spirit he breathed into them through the soul of Islam that exalted themThat raised them out of the lethargy and low level of tribal stagnation.”
  (Islam, Her Moral and Spiritual Value By : Arthur Leonard)

• “We might call him a poet or a prophet, for we feel that the words which he speaks are not the words of an ordinary man. They have their immediate source in the reality of things, since he lives in constant fellows with reality.”
  (Muhmmad by T. Andrae)

• In little more than a year he was actually the spiritual nominal and temporal ruler of Medina With his hand on the lever that was to shake the world.”
  (Muhmmad, the prophet of Allah by: John Austin)

• “Mohamed established his religious system in a manner not only suitable to the sentiments of his compatriots, to their understanding and to the dominating customs of their country, but beyond this , so proportioned to the common ideas of mankind that he concerted more than one half of all human beings to his opinion and all in less than forty years .Thus it seemed sufficient to cause the doctrine to be heard too, thereby subjecting the mind to it.”
  (La vie de Mahomed: Amsterdam by Boulainvilliers)

• “Muhammd (PBUH) is by many seen only through the fog which dread and ignorance have spread around him. To them he is an object of Horror against which anything might be said .But now the mists of prejudice have cleared away. We can afford to see the founder of Islam in a fairer light.
  (The Permanent Elementary in Religion by: Bishop Boyd Carpenter)

• It’s strongly corroborative of muhammad’s sincerity that the earliest coverts to islam where his bosom friends and the people of his house-hold who all intimately acquainted with his private life could not fail to detect those discrepancies which more or less invariably exist between the pretensions of the hypocritical deceiver and his actions at home.”
  (An Apology for Muhammad and the Quran by: John Davenport)

• “... I wanted to know the best of the life of one who holds today indisputable sway over the hearts of millions of mankind. I became more than ever convinced that it was not the sword that won a place for Islām in those days in the scheme of life. It was the rigid simplicity, the utter self-effacement of the prophet, the scrupulous regard for pledges, his intrepidity, his fearlessness, his absolute trust in Allāh and his own mission. Those and not the sword carried everything before them and surmounted every obstacle.”
  (Mahatma Ghandi as quoted in Rahman, op. cit., p. 186)

• “ A man of truth and fidelity, true in what he did, in what he spoke and taught. They noted that he always meant something. A man rather taciturn in speech; silent when there was nothing to be said; but pertinent, wise, sincere, when he did speak; always throwing light on the matter... The word of such a man is a voice direct from Nature’s own heart. Men do and must listen to that as nothing else...”
  (Thomas Carlyle, Quoted in Islām the Religion of all Prophets, 1982, p. 33)