The following quotes of famous non-Muslim personalities have been compiled from numerous sources. On close analysis, one naturally comes to the conclusion that anyone who truly studies this Man is in awe of the person of Mohammad (pbuh).
The following extract has been taken from the book "The 100 - a Ranking of the Most Influential Person in History" by Michael H. Hart, a Christian American, an astronomer, a mathematician, a chess master, and a scientist. After extensive research, he rated Prophet Mohammad (pbuh) as number one and to be considered as the most influential single figure in human history.
"My choice of Mohammad to lead the list of the world's most influential person 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. It is probable that the relative influence of Mohammad on Islam has been larger than the combined influence of Jesus Christ and St. Paul on Christianity. Furthermore, Mohammad (unlike Jesus) was a secular as well as religious leader. In fact, as the driving force behind the Arab conquests, he may well rank as the most influential political leader of all time. It is this unparalleled combination of secular and religious influence, which I feel, entitles Mohammad to be considered the most influential single figure in human history."

Mahatma Gandhi, speaking on the character of Mohammad, (pbuh) says in (Young India):
"I wanted to know the best of one who holds today's undisputed sway over the hearts of millions of mankind. I became more than convinced that it was not the sword that won a place for Islam in those days in the scheme of life. It was the rigid simplicity, the utter self-effacement of the Prophet, the scrupulous regard for his pledges, his intense devotion to this friends and followers, his intrepidity, his fearlessness, his absolute trust in God and in his own mission. These and not the sword carried everything before them and surmounted every obstacle. When I closed the 2nd volume (of the Prophet's biography), I was sorry there was not more for me to read of the great life."

Annie Besant in "The Life and Teachings of Mohammad"
"Mohammad was the soul of kindness, and his influence was felt and never forgotten."

Jules Masserman in "Who Were Histories Great Leaders?" TIME Magazine.
"Head of State as well as Church, he was Caesar and Pope in One; but he was Pope without the Pope's pretensions, and Caesar without the legions of Caesar, without a standing army, without a bodyguard, without police force, without a fixed revenue. If ever a man ruled by right Divine, it was Muhammad, for he had all the powers without their supports. He cared not for the dressings of power. The simplicity of his private life was in keeping with his public life."

Dr. William Draper, M.D. L.L.D. in "History of Intellectual Development of Europe"
"In little more than a year he was actually the spiritual, nominal and temporal ruler of Medina, with his hands on the lever that was to shake the world."

Alphonse de LaMartaine in "Historie de la Turquie"
"The league of nations founded by the Prophet of Islam put the principle of international unity of human brotherhood on such Universal foundations as to show candle to other nations. The fact is that no nation of the world can show a parallel to what Islam has done - the realization of the idea of the League of Nations."

Imam Ja'far ibn Mohammad Al-Sadiq (as) said:
(to one of his companions)
“Verily we are very patient and our Shia’s are even more patient than us.” His companion asked, “May I be your ransom, how can your Shia be more patient than yourselves?” He replied, “Because we are patient alongside our knowledge [of everything], whereas our Shia’s are patient in spite of not knowing.’
[Reference: Usool-e-Kaafi, vol. 2, p. 93]
 

Imām Ja'far ibn Muhammad al-Sādiq (AS) said:

Karbala teaches the humanity a lot of lessons. It is the source of divine enlightenment and a true road to the salvation of mankind.

DON’T ACCEPT HUMILIATION:
In one of his speeches, Imam Hussein (as) said that the authorities in Kufa had given him only two options - humiliation or death. He carried on saying, "And we do not accept humiliation".
A tradition reported by Imam Sadiq (as) records that, other than not accepting humiliation, Allah Almighty has given to believers a choice in all matters.

BROAD-MINDEDNESS:
 

1. Align yourself with right no matter the cost.
2. Enjoin good and forbid evil.
3. Love those who are good and befriend yourself to them.
4. Do not be friends with those who do evil, are cruel or are foolish.
5. When you love someone, you should feel their pain and mourn their tragedies.
6. The more you love someone, the more you will feel for them.
7. You should love God and His messengers and guides more than you love yourself and your family.
8. There is nothing wrong with showing genuine emotion in response to oppression in public.
9. We should all fight oppression with whatever we have.
10. There is more than one way to fight; pick the one that suits the circumstances.

The Brute Ibn e Ziyad at Kufa got the information of Imam Hussain's getting out of the control of Hurr and also learnt that Imam Hussain (as) had camped near the river Euphrates known as the Al-Qama.
On the 3rd of Muharram Ubayd Allah son of Ziyad sent an army counting four thousand led by Umar son of Saad Abi Waqqas towards Karbala. His orders were to get Imam Hussain (as) to surrender to Yazid. Upon Imam Hussain's refusal Umar ibn e Saad asked Ubayd Allah ibn e Ziyad to send more troops. Ubayd Allah ordered all the men in Kufa to come out and join the army to fight against Hussain (as). Anyone not doing so would be killed. Whoever brought the head of Hussain (as) will get a great reward and will be made the Governor of Al-Ray province.

 Think not of those who are slain in God's way as dead. Nay, they live, finding their sustenance in the presence of their Lord.
Holy Quran (3:169)

The events of Karbala reflect the collision of the good versus the evil, the virtuous versus the wicked, and the collision of Imam Hussain (as) versus Yazid. Imam Hussain (as) was a revolutionary person, a righteous man, the religious authority, the Imam of Muslim Ummah (nation). As the representative of his grandfather Prophet Muhammad (pbuh), Imam Hussain's main concern was to safeguard and protect Islam and guide fellow Muslims. On the other hand, the staying power of the rulers (Muawiya and his son Yazid) depended solely on the might of the sword. They used brute force to rule over the Muslim empire even by all possible illicit means.

Karbala's Foundation laid:
The events of history give out a clear verdict that the series of the sufferings, the miseries and the calamities inflicted on the Ahl ul Bait (as), which culminated at one stage in the wholesale massacre of Imam Hussain (as) and his faithful devotees at Karbala, and the misfortunes and the assassinations of the other Imams and their families and their devotees, which thereafter followed up to the eleventh Imam Hasan Al-Askari (as), was inaugurated at Saqeefa. Had the people remembered the repeated declarations about Imam Ali (as), and his Ahl ul Bait (as), the world would never have witnessed the hideous scenes of the sufferings and the massacre of the Holy ones; on the other hand, today the world would have been ruled by the Men of God, and humanity had enjoyed the Godly kingdom on earth, and the human race by this time would have been united under one single brotherhood which was inaugurated by the Holy Prophet (pbuh) at Medina.