Thomas Carlyle wrote: "One other circumstance we must not forget: that he had no school learning; of the thing we call school-learning none at all. The art of writing was but just introduced into Arabia; it seems to be the true opinion that Muhammad never could write! Life in the Desert, with its experiences, was all his education. What of this infinite Universe he, from his dim place, with his own eyes and thoughts, could take in, so much and no more of it was he to know. Curious, if we will reflect on it, this of having no books. Except by what he could see for himself, or hear of by uncertain rumour of speech in the obscure Arabian Desert, he could know nothing. The wisdom that had been before him or at a distance from him in the world, was in a manner as good as not there for him. How he was placed with Kadijah, a rich Widow, as her steward, and travelled in her business, again to the Fairs of Syria; how he managed all, as one can well understand, with fidelity and adroitness; how her gratitude, her regard for him grew: the story of their marriage is altogether a graceful intelligible one, as told us by the Arab authors. He was twenty five; she forty, though still beautiful. He seems to have lived in a most affectionate, peaceable, wholesome way with this wedded benefactress; loving her truly, and her alone. It goes greatly against the impostor theory, the fact that he lived in this entirely unexceptionable, entirely quiet and commonplace way, till the heat of his years was done. Our current hypothesis about Muhammad, that he was a scheming Impostor, a Falsehood incarnate, that his religion is a mere mass of quackery and fatuity, begins really to be now untenable to anyone. The lies, which well-meaning zeal has heaped round this man, are disgraceful to ourselves only... It is really time to dismiss all that. The word this man spoke has been the life-guidance now of a hundred and eighty millions of men these twelve hundred years. These hundred and eighty millions were made by God as well as we. A greater number of God's creatures believe in Muhammad’s word at this hour, than in any other word whatever. Are we to suppose that it was a miserable piece of spiritual legerdemain, this which so many creatures of the Almighty have lived by and died by?"
A French philosopher Lamartime writes in his book ‘History of Turkey’: "If greatness of purpose, smallness of means, and outstanding results are the three criteria of human genius, who could dare to compare any great man in modern history with Muhammad? The most famous men created arms, and empires only. They founded, if any at all, no more than material power which often crumbled away before their eyes. This man merged not only armies, legislation, empires, peoples and dynasties but millions of men in one third of the inhabited world, and more than that, moved the altars, the gods, the religions, the ideas, the beliefs and the souls on the basis of a Book, every letter of which has become law. He created a spiritual nationality of every tongue and of every race. Philosopher, orator, apostle, legislator, warrior, conqueror of ideas, restorer of rational dogmas, 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 ask, is there any man greater than he?"
John Devonport wrote: "Is it possible to conceive, we may ask, that the man who effected such great and lasting reforms in his own country by substituting the worship of the one only true God for the gross and debasing idolatry in which his countrymen had been plunged for age" [who brought about great, enduring reformations]" to have been a mere impostor, or that his whole career was one of sheer hypocrisy? No, surely, nothing but a consciousness of really righteous intentions could have carried Mohammed so steadily and constantly without ever flinching or wavering, without ever betraying himself to his most intimate connections and companions, from his first revelation to… to his last… So much so that it may be affirmed with certain truth, that if the Western princes had been lords of Asia instead of the Saracens and Turks, they would not have tolerated Muhammadanism as Muhammadans have tolerated Christianity, since they persecuted, with the most relentless cruelty, those of their own faith whom they deemed heterodox."
Michael Hart: "My choice of Muhammad to lead the list of the world's most influential persons may surprise some readers and may be questioned by others, but he was the only man in history who was supremely successful on both the religious and secular levels. ... On the purely religious level, then, it seems likely that Muhammad has been as influential in human history as Jesus."
Karen Armstrong writes in her book ‘Muhammad - A Biography of the Prophet’: "Muhammad had to start virtually from scratch and work his way towards the radical monotheistic spirituality of his own. When he began his mission, a dispassionate observer would not have given him a chance… . In fact, to attempt to introduce it on a large scale in this violent, terrifying society could be extremely dangerous and Muhammad would be lucky to escape with his life. Indeed, Muhammad was frequently in deadly peril and his survival was a near-miracle. But he did succeed. By the end of his life he had laid an axe to the root of the chronic cycle tribal violence that afflicted the region and paganism was no longer a going concern. The Arabs were ready to embark on a new phase of their history. Finally it was the West, not Islam, which forbade the open discussion of religious matters. At the time of the Crusades, Europe seemed obsessed by a craving for intellectual conformity and punished its deviants with a zeal that has been unique in the history of religion. The witch-hunts of the inquisitors and the persecution of Protestants by the Catholics and vice versa were inspired by abstruse theological opinions which in both Judaism and Islam were seen as private and optional matters. Neither Judaism nor Islam share the Christian conception of heresy, which raises human ideas about the divine to an unacceptably high level and almost makes them a form of idolatry."
Annie Besant writes in her book 'The Life and Teachings of Muhammad': "It is impossible for anyone who studies the life and character of the great Prophet of Arabia, who knows how he taught and how he lived, to feel anything but reverence for that mighty Prophet, one of the great messengers of the Supreme. And although in what I put to you I shall say many things which may be familiar to many, yet I myself feel whenever I re-read them, a new way of admiration, a new sense of reverence for that mighty Arabian teacher."
Ruth Cranston wrote: "Muhammad (peace and blessings of Allah be on him) never instigated fighting and bloodshed. Every battle he fought was in rebuttal. He fought in order to survive…and he fought with the weapons and in fashion of his time… Certainly no ‘Christian’ nation of 140,000,000 people who today dispatch (this is a book written in 1949) 120,000 helpless civilians with a single bomb can look askance at a leader who at his worst killed a bare five or six hundred. The slayings of the Prophet of Arabia (peace and blessings of Allah be on him) in the benighted and bloodthirsty age of the seventh century look positively puerile compared with our own in this ‘advanced’ and enlightened twentieth. Not to mention the mass slaughter by the Christians during the Inquisition and the Crusades – when, Christian warriors proudly recorded, they 'waded ankle-deep in the gore of the Muslim infidels.'"