Abu Sarruk said to Imam Jafar-e-Sadiq^-asws that we present Quranic Verses to our opponents (in beliefs) to prove they are on the unjust path. When we present (4:59):

يَا أَيُّهَا الَّذِينَ آمَنُوا أَطِيعُوا اللَّهَ وَأَطِيعُوا الرَّسُولَ وَأُولِي الْأَمْرِ مِنْكُمْ

They say that the ‘Ulil al-Amr’ are the high rank rulers, we present another Verse as proof,

إِنَّمَا وَلِيُّكُمُ اللَّهُ وَرَسُولُهُ وَالَّذِينَ آَمَنُوا الَّذِينَ يُقِيمُونَ الصَّلَاةَ وَيُؤْتُونَ الزَّكَاةَ وَهُمْ رَاكِعُونَ (5:55)

They say that this Verse is for ordinary momineen, we present the Verse of Mowaddah:

قُلْ لَا أَسْأَلُكُمْ عَلَيْهِ أَجْرًا إِلَّا الْمَوَدَّةَ فِي الْقُرْبَى ٌ (42:23)

They say that ‘Filqurab’ means the near relatives of momineen. I knew other Verses of Quran and presented to them but they came up with similar unrealistic justifications.

Imam^-asws said: You should asked them for the ‘Mubahila[1]‘ upon encountering this situation. I asked how could I do ‘Mubahila’?

Imam^-asws replied: you should, first, spend three days to cleanse your soul through ‘Ibadah’, keep fast and invite your opponent to a higher elevation (i.e., hill), upon reaching there you should put his right-hand fingers into your right hand and try to be fair with him and reciting first as: Ya Allah^-azwj Who is the Lord of the Earth and Heavens, the One who knows about the present and what to come. The most kind and the most Merciful, if this person has denied the ‘Just’ and supported the ‘unjust’, then please send down you wrath on him from the skies what he deserves or hit him with painful tragedy. And then ask your opponent to repeat the same words that if this person has denied the right and favoured the unjust then send down Your^-azwj wrath onto him and strike him with calamities. Thus you will see him in immense troubles without any prior signs of them. The narrator says, by my Lord, no one accepted my Mubahila offer when I proposed it to others.[2] In another Imam Mohammed Baqir^-asws says the time for Mubahila is between the Fajr till the sunrise (a tradition of Mohammed Baqir^-asws also presents the same tradition).[3]

[1] Sending Allah^azwj’s curse on the lair, as per the Verse of Mubahila

فَمَنْ حَاجَّكَ فِيهِ مِنْ بَعْدِ مَا جَاءَكَ مِنَ الْعِلْمِ فَقُلْ تَعَالَوْا نَدْعُ أَبْنَاءَنَا وَأَبْنَاءَكُمْ وَنِسَاءَنَا وَنِسَاءَكُمْ وَأَنْفُسَنَا وَأَنْفُسَكُمْ ثُمَّ نَبْتَهِلْ فَنَجْعَلْ لَعْنَةَ اللَّهِ عَلَى الْكَاذِبِينَ (61 3)

[2] Usool-e-Kafi, vol. 5, ch.34, tradition 1.

[3] Usool-e-Kafi, vol. 5, ch.34, tradition 2.