From The Abridged Tafseer of Ibn Katheer
Aali Imraan 159
And by the mercy of Allah, you dealt with them gently. And had you been severe and harsh-hearted, they would have broken away from about you; also pardon them, and ask forgiveness for them; and consult them in the affairs...
Among the Qualities of Our Prophet Muhammad are Mercy and Kindness
Allah addresses His Messenger and reminds him and the believer of the favor that He has made his heart and words soft for his Ummah, those who follow his command and refrain from what he prohibits.
And by the mercy of Allah, you dealt with them gently...
meaning, who would have made you this kind if it was not Allah mercy for you and them?
Qatadah said that, And by the mercy of Allah, you dealt with them gently... means, "with Allah's mercy you became this kind."
Al-Hassan Al-Basri said that this indeed is the description of the behavior that Allah sent Muhammad with. This aayah is similar to Allah's statement, Verily there has come unto you a Messenger from among yourselves. It grieves him that you should receive any injury or difficulty. He is anxious over you (to be rightly guided, to repent to Allah); for the believers he is full of pity, kind, and merciful (9:128)
And had you been severe and harsh-hearted, they would have broken away from about you; (5:159)
The severe person is he who utters hash words and "harsh-hearted" is the person who heart is hard. Had this been the Prophet's behavior, "they would have scattered from around you." However Allah gathered them and made you kind and soft with them, so that their hearts congregate around you."
'Abdullah Ibn 'Amr (radiallahu 'anhu) said that he read the description of the Messenger of Allah in previous Books, "He is not severe, harsh, obscene in the marketplace or dealing evil for evil. Rather he forgives and pardons." [Fathul-Bari 8:449]
Verily there has come unto you a Messenger from among yourselves. It grieves him that you should receive any injury or difficulty. He is eager for you; for the believers he is full of pity, kind and merciful. But if they turn away say: "Allah is sufficient for me. There is no God but He, in Him I put my trust and He is the Lord of the Mighty Throne. [At-Tawbah 128 - 129]
This is similar to His other statement: And be kind and humble to the believers who follow you. Then if they disobey you, say: "I am innocent of what you do." And put your Trust in Allah the All-Mighty, the Most Merciful. [26:215 - 217] -
From The Ideal Muslim
The true Muslim is sincere towards Allah his Book, His Prophet (sallallaahu 'alaihi wa sallam) and to the leaders and the masses of the Muslims as is stated in the hadeeth: The Prophet (sallallaahu 'alaihi wa sallam) said, "Religion is sincerity." We asked, 'To Whom?' He said, "To Allah (by obeying Him attributing to Him what He deserves and performing jihaad for His sake); to His Book (by reading it, understanding it and applying it to one's daily life); to His Prophet (by respecting him greatly and fighting on his behalf both in his lifetime and after his death, and by following his sunnah); to the rulers of the Muslims (by helping them in their task of leading Muslims to the right path and alerting them if they are heedless); and to their common folk (by being merciful towards them). [Al-Bukhaari and Muslim)
It is no surprise, then, that the Muslim should be sincere towards his brothers and not cheat them or mislead them. Sincerity in this sense is one of the most basic principles of Islam which the first believers pledge to adhere to when they gave allegiance to the Prophet (sallallaahu 'alaihi wa sallam). This is confirmed by the statement of Jarir Ibn Abdullah; "I gave allegiance to the Prophet (sallallaahu 'alaihi wa sallam) and pledged to observe regular prayer, to pay zakat and to be sincere towards every Muslim." (Al-Bukhari and Muslim) "
In the hadeeth quoted above, we see that the Prophet (sallallaahu 'alaihi wa sallam) summed up Islam in one word, naseehah, showing that sincerity is the central foundation of the faith. For without sincerity a man's faith is invalid and his Islam is worthless. This is the meaning of the hadeeth of the Prophet (sallallaahu 'alaihi wa sallam): None of you truly believes until he likes for his brother what he likes for himself (Al-Bukhaari and Muslim) This is impossible to achieve unless one loves one's brother with all sincerity. No doubt this level of love for one's brother is very difficult to attain, but it is not impossible as long as one is constantly aware that liking for one's brother what one likes for oneself is one of the conditions of faith, and that religion is sincerity. Indeed it is the natural attitude of the sincere Muslim who truly understands Islam