Islam has spread much more quickly than the other religions all over the world. The question is how this has happened? And it becomes more important when we consider the situation in which Prophet Muhammad (PBUH&HP) emerged.

He (PBUH&HP) invited people to Islam while he did not have any wealth or position and even when his family opposed him; except his wife, Lady Khadija (AS), and his cousin, Imam Ali (AS). Several factors were involved which we will discuss in a series of articles. Here, the focus is on how pleasant characteristics of Prophet Muhammad (PBUH&HP) helped Islam to become well known throughout the world.

The Quran confirms the impact of Prophet Muhammad’s (PBUH&HP) behavior 

In the Quran it is emphasized that the morals and attitude of Prophet Muhammad (PBUH) was one of the factors which attracted people to Islam: “had you been harsh and hard-hearted, they (people) would have surely scattered from around you” (3:159). This pleasant behavior of the Prophet (PBUH&HP) which made people inclined to him and Islam is known to be due to the God’s mercy over him (PBUH&HP) (3:159).

Indulgent in Personal Affairs, Severe in People’s Rights

Prophet Muhammad (PBUH&HP) was forgiving towards those who had bothered him or even insulted him. It is narrated that once, a Jew accused Prophet (PBUHHP) of owing him some money and insisted on paying him back immediately. Prophet (PBUH&HP) told him gently that he had made a mistake, but the man repeated his claim and suddenly assaulted the Prophet (PBUH&HP). The companions of Prophet (PBUH&HP) came to interfere, but he (PBUH&HP) said: “I know how to deal with my friend.” He (PBUH&HP) behaved very nicely such that the man acknowledged that this behavior is only expected from a prophet and he declared his belief and embraced Islam right there. 

However, he (PBUH&HP) was very serious about applying Islamic regulations or others’ rights. It is narrated that a woman from Quraysh [i] had stolen something; she confessed and had to be punished. Many people, including her family, interfered to change the Prophet’s mind and to stop her punishment. Prophet (PBUH) told people that they had meddled because she was from a famous family, but there is no exception to Islamic rules. 

Taking Advice from Others

To provide an exemplary leader for others, to train people to act alike, to valorize his companions and to convince people to follow his instructions, Prophet (PBUH) used to take advice. Of course, in the end, he (PBUH&HP) was the one who decided and made firm decisions: “consult them on the matter. And when you have decided, then rely upon Allah” (3:159).

Advertising and Inviting to Islam with Wisdom and Affection by Prophet Muhammad (PBUH&HP)

“Invite (mankind) to the Way of your Lord with wisdom and fair preaching, and argue with them in a way that is better” (16:125). Unlike what is advertised against Islam, Prophet Muhammad (PBUHHP) never forced people or threatened them to accept Islam.

On the contrary, following the advice of the Quran, he promoted Islam with kindness, fairness, and wisdom. Instead of humiliating and discriminating non-Muslims, or being harsh and fierce, he used rational argument and discussion which also improved their rationality and thinking power [1]. This followed the fact that embracing Islam should be according to the personal perception and understanding and it cannot be based on the imitation [2].

Notes:
[i]A mercantile tribe that historically inhabited and controlled Mecca and Ka'aba.

References:

1. Spreading Islam 

Imam Jawad (AS) is the ninth Shiite Imam. His father is Imam Reza (AS), and his mother’s name is  Sabikah. His original name is “Muhammad,” but his epithet is “Jawad,” meaning “a very generous person.” He was born on 195 H (810 AC) in Medina. (1)

Generally, Shiite Imams carried prestige among their tribe and were respected by everyone. Many Shias believed that Imams were merely knowledgeable people who gained their knowledge only from their ancestors and they were not extraordinary. However, the imamate of Imam Jawad (AS) made all the Shias think differently and change their minds. He was only eight years old, when his father, Imam Reza (AS), was martyred. According to hadiths, the earth cannot be without an Imam. Accordingly, Imam Reza (AS) said:
The earth cannot be without any of us, Imams, whether he is known by the people or hidden from them. (2)

Shias didn’t know who would be their leader after Imam Reza (AS). But, he introduced Imam Jawad (AS) as his successor (3) and showed the Shias that “Imam” was not an ordinary person and he could be the leader of the whole Shias despite being young. 

Knowledge is a sign of imamate 

The hadiths suggest various signs for Imams so that we can differentiate between the truly chosen Imams by Allah and impostors. One of the most significant signs is that he is the most knowledgeable person of his time. 

Prophet Muhammad (PBUH & HP) said:
Do not try to teach something to Ahlul Bayt because they are far more knowledgeable than you. (4)
Regarding this, Imam Sadiq (AS) acknowledged:
Indeed Ali (AS) was a knowledgeable person whose knowledge will be passed to his successors and no knowledgeable person (an Imam) dies except to leave their knowledge to someone else. (5)
He also said:
The successor of an Imam attains the knowledge of the previous Imam in the last moment of his life. (6)

The debates of Imam Javad (AS) with religious scholars

Ma’mun was one of the Abbasid Caliphs who lived during the same period as Imam Reza (AS) and Imam Jawad (AS). During his caliphate, the Shia population increased dramatically which intimidated him. Due to this fact, he lied that Imam Reza (AS) was his successor and led people to believe that he was not an enemy of Shiite Imams. But at last, he killed Imam Reza (AS). After Imam Reza (AS), Imam Jawad (AS) was his most important enemy. He made Imam Jawad (AS) marry his daughter so that he could deceive people longer. 

Many of the Abbasid royalties were against Ma’mun’s decision, as they believed that the boy was too young and he wasn’t a knowledgeable person. Ma’mun proposed that they should test the knowledge of Imam Javad (AS), in order to show everyone how extraordinary he is. Imam Javad (AS) was only nine years old at that time. 

One of the most knowledgeable religious scholars in those times, Yahya ibn Aktham, volunteered to have a discussion and debate with Imam Javad (AS). 
During the discussion, Yahya ibn Aktham asked Imam Javad (AS) about a person who has killed an animal while making Hajj; what should he do? (During the Hajj ceremony, it is forbidden for the pilgrims to kill any creature).
Imam Jawad (AS) said that it depends: Did he kill it in Mecca or outside of it? Did he know about this law or not? Has he done it intentionally or unintentionally? Was he free or was he a slave? Was it his first time? Was it a bird or something else? Was it a small animal or a big one? Is he ashamed of what he did or not? Did he do it at night or during the day? Was it Umrah (a special type of Hajj) or regular Hajj? 
Yahya was so surprised that he couldn’t answer the twelve questions of Imam Jawad (AS), but instead, Imam Javad (AS) answered them one by one and explained the rules to everyone. 
Ma’mun told people that “Now do you see what I was telling you?” (7)
During another session, Yahya asked Imam Jawad (AS) about a hadith from the Prophet (PBUH & HP) that Sunni narrators had recorded. The hadith said “Once Gabriel came to the Prophet (PBUH & HP) and told him ‘O Muhammad! Allah told you to inform Abu-Bakr that I am satisfied with him. Is he satisfied as well?” Yahya questioned Imam Javad (AS) about the authenticity of this hadith.
Imam Jawad (AS) said, “I do not deny the privileges of Abu-Bakr … but this hadith is contrary to the verses of the Quran.” Imam Javad (AS) stated he wouldn’t deny the privileges of Abu-Bakr because he was surrounded by Sunnis and it was so dangerous for him to speak out against Abu-Bakr.
Then he said, “The holy Quran says 
‘Surely We have created man, and We know the promptings of his heart, and We are nearer to him than even his jugular vein.’ (50:16) 
Then why should Allah ask Abu-Bakr whether he is satisfied with him or not while Allah already knows it?”
Yahya referred to another hadith that claims Abu-Bakr and Umar on the earth are like Gabriel and Michael in the skies. 
Imam Javad (AS) said “This hadith cannot be accepted as well, because Gabriel and Michael never worshiped anyone except Allah, while Abu-Bakr and Umar spent most of their lives as polytheists and worshippers of idols.” 
Yahya added that Abu-Bakr and Umar are the lords of the elderly in Paradise.
Imam Javad (AS) stated that it is impossible as people in Paradise are all young not old. Umayyad dynasty made this fake hadith because they didn’t like the authentic one narrated by the Prophet (PBUH & HP) that reads, “Hassan (AS) and Hussain (AS) are the lords of young people in Paradise.” (8)
Yahya said it also has been narrated that Umar is the one who illuminates Paradise.
Imam Javad (AS) responded that it is also impossible. Do you think that Paradise is not illuminated by the Angles and Prophets like Adam (AS) and Muhammad (PBUH & HP) but rather by Umar?!
Yahya said it has been claimed that the angels talk with the tongue of Umar.
Imam Jawad (AS) said that he does not deny the privileges of Umar. However, Abu-Bakr was better than Umar as he said, “I have a Satan that sometimes tempts me, so if you see me deviating, stop me.” 
Yahya asked many other questions about Abu-Bakr and Umar, and Imam Javad (AS) answered them all. (9)   
Not only Imam Javad (AS) did not fail to answer the questions, but also, he posed more questions that even the most knowledgeable scholars felt incapable of disputing with Imam Javad (AS). These debates happened when Imam Javad (AS) was only 8 or 9 years old. 
As it might be apparent, Shiite Imams were extraordinarily knowledgeable, and their knowledge was given to them by God not by learning in classes and the like. They were the most knowledgeable individuals in their era regardless of their age.

Resources

1.  I’lam al-Vara, Sheikh at-Tabarsi, vol.2, pg.91
2.   Kamal ad-Din, Sheikh Saduq, vol.1, pg.202
3.  Al-Kafi, Sheikh Koleini, vol.1, pg.320
4. Mu’jam al-Kabir, at-Tabarani, vol.5, pg.166
5.  Al-Kafi, Sheikh Koleini, vol.1, pg.221
6.  Al-Kafi, Sheikh Koleini, vol.1, pg.275
7.  Al-Ih’tijaj, at-Tabarsi, vol.2, pg.444
8. Sunan al-Kubra, Nasa’i, vol.7, pg.460
9. Al-Ih’tijaj, at-Tabarsi, vol.2, pg.446-448