It was the 10th year of Hijrah, the religion of Islam had now become well-established in the Arabian subcontinent. The Prophet (s), however, was still determined to spread the religion of Islam even further, as was his duty ordained by Allah (swt). It was not unusual for him to write letters and send representatives to distant and foreign tribes in an effort to bring them to Islam. One particular incident was his invitation to the Christians of Najran (modern-day Yemen). He clearly stipulated that the tribe had two options: to either accept Islam or to continue practicing Christianity but pay jizya (tax) to the Muslim government. Instead of accepting one of the two options like most other tribes, the Christians of Najran chose to travel to Medina to plead their case.
The basis of their argument was the case of Prophet Isa (Jesus); the Christians insisted that because he was born without a father, then certainly his father could be none other than God Himself. In response to this, a verse of the Holy Qu’ran was revealed:
“Indeed the case of Jesus with Allah is like the case of Adam: He created him from dust, then said to him, ‘Be,’ and he was.” (3:59)
This was a significant point. If Prophet Isa (s) not having a father made him the son of God, then how about Prophet Adam (s), who had neither father nor mother? However, the Christians remained unconvinced. They still doubted other tenets of Islam, such as the divine Prophethood of Prophet Muhammad (saw) and the concept of the oneness of Allah (swt). When both parties were unable to reach a satisfactory conclusion, they resorted to the Arab tradition of Mubahila. Mubahila comes from the Arabic word “bahla” (بهلة) which means to mutually curse.  Both parties would invoke God’s curse upon the party that was in error. The idea was that this practice would effectively expose the people of falsehood.This decision was initiated by the Prophet (s) after the revelation of this verse:
“Should anyone argue with you concerning him (Jesus), after the knowledge that has come to you, say, ‘Come! Let us call our sons and your sons, our women and your women, our souls and your souls, then let us pray earnestly, and call down Allah’s curse upon the liars.’” (3:61)
It was the direct command of Allah (swt) to bring the argument to an end in this manner, as this was the most effective way to distinguish truth from falsehood. The Christians, of course, upon hearing this, became hesitant to go through with the argument. When they saw that Prophet Muhammad (saw) had bought with him those most beloved to him, his own flesh and blood, his daughter, Sayyida Fatima (as), her husband, Imam Ali (as), and their two sons, Imam Hassan and Hussain (s), they were in complete shock. Notable indeed are the words uttered by the chief Bishop of the Christian tribe upon seeing this group: “By Jesus! I see the faces that if they turn upward to the heavens and pray, the mountains shall move. Believers in Jesus of Nazareth, I tell you the truth. Should you fail to come to some agreement with Muhammad, he, along with the godly souls with him, shall wipe out your existence forever, should they invoke the curse of God on you.”  Thus, the Christians surrendered, agreeing to live under the Muslim government while paying a tax and the Muslims emerged victorious.
The event of Mubahila was a manifestation of the truth of Islam over all other religions. But the event was not just to show the might of Islam to those who were non-Muslim. It also served a deeper purpose, and that was to show the Muslims the true status of the AhlulBayt (s). In correspondence with verse 61 from Surah Ale-Imran mentioned above, Prophet Muhammad (s) chose Sayyida Fatima (as) to represent the women of Islam, chose his grandchildren Imam Hassan and Hussain (s) to represent his sons, and most interestingly, chose Imam Ali (as) to represent his own nafs (soul/self). This was just one of the many incidents reminding the Muslims of who their leader would be after the passing of the Prophet (s). In this event, Imam Ali (as) is considered to be an extension of the Holy Prophet (s) himself, as he is referred to being the nafs of the Prophet (s) in the Qur’anic verse.
It is tragic that just months later, the Prophet (s) passed away, and the Muslims seemed to have forgotten why the Prophet (s) specifically chose Imam Ali (as) to represent himself at this incident and not anyone else. The Prophet (s) did not have to exclusively bring his family members. He could have chosen companions or other community members. In fact, there were many companions anxiously waiting, assured that they would be the ones chosen by the Prophet (s) . But who the Prophet (s) chose to represent Islam pointed to a stark reality: there was no one better acquainted with the reality of Islam and better suited to spread its light than these individuals. After the Prophet (s) himself, these people had the most thorough understanding of the religion. They represented the true face of Islam as the religion of righteousness. Their mere presence caused all other religions (in this case, represented by the religion of Christianity) to pale in comparison. These humans were epitomes of perfection and pinnacles of truth and justice in the face of falsehood and injustice. They were chosen by the Prophet (s) himself to represent the pure religion of Islam, as witnessed by the divine event of Mubahila.
 An Enlightening Commentary into the Light of the Holy Quran, Volume 3; courtesy of www.al-islam.org
 The Event of Mubahila (with excerpts from commentary of Agha Mehdi Pooya); courtesy of www.islamicthought.co.uk
 Event of Mubahila from Tafsir al-Mizan; courtesy of www.duas.org