The delegation questioned Muhammad about the nature of Jesus. The next day, he returned with this God-given verse:
The likeness of Jesus before God is as the likeness of Adam. He created him of dust, then he said to him, ‘Be!’ and he was. This is the truth from your Lord, so be not of the disputers. And if anyone disputes with you after that knowledge has come to you, then say: ‘Come let us call our sons and your sons, and our women and your women, and ourselves and yourselves, then let us pray earnestly, and pray for the curse of God on the liars. — Qur’an 3.59–61
Basically, that Jesus is just as Adam in God’s sight, a created prophet, and that the dispute should be settled in prayer, calling on God to judge the liar with a curse. The Najranites agreed to this spiritual combat.
On the day set (24 Dhul Hijja), Muhammad came out carrying Hussain, with Hassan holding his finger, and Fatima and Ali walking behind. When the Najranites saw the holiness glowing in their faces they chose not to pray against them, settling to pay jizya each year. Thus, according to Islamic sources, Islam proved itself against Christianity.
It’s a bit one-sided to leave there, especially because I’m Christian. The Christians of Najran had a long history of inter-religious conflict, most famously suffering massacre in the sixth century.
During the sixth century, Arabia Felix (Yemen) was a major border area between the Byzantine and Persian spheres of influence. Close ties with Ethiopia and the spread of Judaism through both regions created a complex demographic. The conversion to Christianity of Negus Caleb of Ethiopia tipped the balance in favour of the growing Christian community in Ethiopia and southern Arabia. The Najranite persecution is placed firmly on the shoulders of Dhu Nuwas, renamed Yusuf Asar, usurper King of the Himyarites, and convert to Judaism. His anti-Christian policies, and siege of Ethiopian outposts in the coastal areas of his kingdom, led to the amphibious assault of southern Arabia by the Negus’s troops, which led to the short-lived dominance of the Christian religion in pre-Islamic southern Arabia. There are a number of Himyarite and Ethiopic inscriptions that support the general background historicity. And the polemical writings of Simeon of Beth-Arsham, preserved in Syriac and Garshuni (Arabic in Syriac script), give us the most full account of the martyrdom of the Christians at the hands of Yusuf Asar Dhu Nuwas (and there’s one really good tale in there, that I should tell sometime).
Islam came into an Arabia that was being torn apart by bigger powers, and religion was an integral part of their political games. Where Yusuf Asar Dhu Nuwas’s religious revolution failed in its brutality, Muhammad’s religious revolution was successful in its reasoned arguments and prayerfulness. May we live in peace, and God judge the right.