You asked about belief.
If you can be fair then I can tell you about belief.
For what is it worth to believe if you cannot be fair.
Moreover, the greatness of a man is not measured by how much he is fair to his friends, but by the way he is fair to his foes.
But to go beyond fairness, that is to step towards mercy, belongs to a true believer.
It is not a claim, but a state of being. “By their fruits, you shall know them” as it is written.

There is a lot of sound records that Muslims hold on the Prophet Muhammad, peace be upon him.
It is truly a wealth for a seeker.
For instance, dealing with a foe, it has been related by Imam Muslim [born ~206 Hijra/ ~ 815 CE – died 261 Hijra / 875 CE ] and others that Muhammad, the Messenger of Allah, peace be upon him, sent some cavalry towards Najd [East of Arabia], and they brought a man of Banu Hanifah [a tribe who dwell in Najd] who was called Thumamah bin Uthal, the leader of the people of Yamamah. They tied him to one of the pillars of the mosque, then the Messenger of Allah came out to him and said:
“What do you have to say, O Thumamah?”
He said: ” 0 Muhammad, I shall say something good. If you kill me, you will kill one who has shed blood, and if you show me kindness, you will be showing kindness to one who is grateful, and if you want money, then ask, and you will be given whatever you want.”
The Messenger of Allah left him alone until the next day, then he said:
“What do you have to say, O Thumamah?”
He said: “What I said to you: If you show me kindness, you will be showing kindness to one who is grateful, and if you kill me, you will kill one who has shed blood, and if you want money, then ask, and you will be given whatever you want.”
The Messenger of Allah left him alone until the next day, then he said:
“What do you have to say, O Thumamah?”
He said: “I say what I already said to you: If you show me kindness, you will be showing kindness to one who is grateful, and if you kill me, you will kill one who has shed blood, and if you want money, then ask, and you will be given whatever you want.”
The Messenger of Allah. said:
“Let Thumamah go.”
So he went to some date palms that were near the mosque, and washed himself, then he entered the mosque and said: “I bear witness that none deserves to be worshipped but Allah and I bear witness that Muhammad is His slave and Messenger. O Muhammad, by Allah, there was no face on earth that was more hateful to me than your face, but now your face has become the dearest of all faces to me. By Allah, there was no religion on earth that was more hateful to me than your religion, but now your religion has become the dearest of all religions to me. By Allah, there was no city on earth that was more hateful to me than your city, but now your city has become the dearest of all cities to me. Your cavalry seized me when I was intending to do Umrah [a short and light ritual when visiting Makkah]. What do you think?”
The Messenger of Allah gave him glad tidings and told him to do Umrah.
When he came to Makkah, someone said to him: “Have you changed your religion?”
He said: “No, but I have submitted myself with the Messenger of Allah and no, by Allah, no grain of wheat will come to you from Yamamah unless the Messenger of Allah gives permission. ”

“And We have not sent you, except as a mercy to the worlds.”
َQuran 21:107