There is no room here to trace how Christianity was disseminated in Nejran; suffices to say that the Ethiopian invasion of southern Arabia introduced Christianity there for the first time. The incident of Abraha and his elephant demonstrates the fact that they were quite fanatical in their religious fervor.
In 630 A.D., Prophet Muhammad sent a letter to Nejran's bishop the contents of which are narrated by the grandson of its recipient, namely Salamah ibn Abd Yashu’, as follows: In the Name of Allah, the God of Abraham, Isaac, and Jacob, from Muhammad, the Messenger of Allah, to the Bishop of Nejran and to the people of Nejran: If you accept Islam, I shall praise on your behalf the God of Abraham, Isaac and Jacob. I invite you to worship Allah rather than to worship the servants of Allah, and I invite you to accept the authority of Allah rather than the authority of His servants. But if you refuse, you will have to pay the jizya. And if you refuse to pay the jizya, then I warn you of war. Wassalam.[1]
Having read this letter, the bishop was terrified. He called a man of Nejran named Shurhabil ibn Wada’ah to his presence, passed the Prophet's letter to him to read, then asked him what he had to suggest. Shurhabil said, “Do you know of the promise which Allah had made to Abraham with regard to the offspring of Ishmael, that they will have a prophet rising from among them, so could this man be he? I cannot pass a judgment about Prophethood, but if you ask me about anything relevant to life's matters, I can make suggestions to you.’ The bishop then solicited the suggestion of one man from Nejran after another, and the answer was the same. Finally, they decided collectively to send Shurhabil ibn Wada’ah, Abdullah ibn Shrhabil, and Jabbar ibn Fayz to meet the Prophet and to report to them their impressions of him. These are the same men known as the ‘Aqib, Sayyid, and al-Ahtam, namely Abul-Harith, respectively.
They were considered to be leaders in all affairs, and the were joined by eleven more, making their total number fourteen. When the deputations reached Medina, they dressed themselves in silk garments, put on gold rings then went to the mosque. All of them greeted the Prophet traditionally, but the Prophet did not respond, turning his face away from them.
They left the mosque and approached ‘Uthman and ‘Abdul-Rahman ibn ‘Awf complaining thus: “Your Prophet wrote us inviting us here, but when we came to him and greeted him, he neither reciprocated our greeting nor said a word to us. Now what do you advise us to do? Should we go back or wait here?’ ‘Uthman and ‘Abdul-Rahman ibn ‘Awf sought Ali's advice. Ali said, “These people should first remove the silk clothes and gold rings. Then they should go and see the Prophet.’
When they did as they were advised, the Prophet responded to their greetings and said, “By the Lord Who has appointed me as His own Messenger, when they first came to me, they were accompanied by Satan.’ Thereafter, the Prophet preached to them and invited them to accept Islam. They asked him: “What is your opinion about Jesus?’ The Prophet said, “You may rest today in this city and, after being refreshed, you will receive the reply to all your questions from me.’
On the next day, the Prophet recited to them these Qur'anic verses: Surely the likeness of ‘Isa (Jesus) is with Allah as the likeness of Adam: He created him from dust then said to him, ‘Be', and he was. The truth is from your Lord, so you should not be of those who doubt (it).(Qur'an, 3:59-60)
They did not accept the words of the Lord and insisted on their own belief. Then the following verse was revealed: But whoever disputes with you in this matter after what has come to you of the knowledge, say: Come! Let us call our sons and your sons, and your women and our women, and ourselves and yourselves, then let us pray earnestly and bring about the curse of Allah on the liars.(Qur'an, 3:61)
They sought a day's respite and privately solicited ‘Aqab's advice. He said, “By God! You know that Muhammad is the Messenger of the Lord and that he has given a clear and appreciable verdict. Do not enter into a maledictory trial with him or else you should be destroyed. If you wish to remain adhering to your religion, accept to pay the jizya and make a pact.’ On the next day, therefore, they came out on one side and on the other the Prophet came out of his house carrying Husain in his arms as Hasan was walking by his side holding his finger. Behind him was Fatima and behind her Ali. Praised be to Allah! What a time it was! What an atmosphere! How good a witness and how glorious the witnessed!
In short, the Prophet confronted the Christian delegates and said to Hasan, Husain, Fatima and Ali: “When I curse them, you say ‘Amen' together.’ When the Christians saw the five holy Purified ones, they were awe-stricken. Abul-Harith, who was their wisest, said, “My people! At this moment, we are looking at such personalities that if they pray to God, they can move mountains. Abstain from this maledictory conflict (Mubahilah) or else you should be destroyed and no Christian will remain on the face of the earth.’
They pleaded to the Prophet thus: “O Abul-Qasim! We shall not have a malediction with you.’ The Prophet invited them to accept Islam. They declined again and said that they were prepared for a treaty that they would present every year two thousand pieces of garments costing 40 dirhams each. According to another tradition, it is said that they also agreed to give 30 horses, 30 camels, 30 coats of mail and 30 spears every year. Thus, a settlement was made.
When the Christians of Najran declined from resorting to a maledictory conflict against the Prophet, he said, “By the Lord Who has appointed me His Messenger in truth, had they chosen the malediction, there would have been a shower of fire upon them in this very field.’ Jabir says, “The verse (Chap. 3, verse 61) was revealed in reference to this contest. In this verse, the word “selves’ refers to the Prophet and Ali; the word “sons’ refers to Hasan and Husain, and the word “women’ refers to Fatima.’
In Tabari's Tarikh, it is stated that during the 10th year of Hijra, the Prophet sent Ali to Yemen. Prior to that, he had sent Khalid ibn al-Walid in order to call the people of Yemen to Islam, but nobody accepted Islam. Then the Prophet sent Ali and authorized him that he might, if he so desired, dismiss Khalid or anyone else from his party. So, Ali went to Yemen and read the Prophet's statement to the people there. As a result, in one day, all members of the clan of Hamadan were converted to Islam. Ali informed the Prophet of this success whereupon the Prophet said, “Peace be upon the Hamdanites!’ Thereafter, all Yemenites entered into the folds of Islam. Ali again informed the Prophet of the progress which he had made. The Prophet was so elated, he offered a sajda (prostration) to thank Allah.
During this year, the Prophet deputed Ali to go to receive the jizya from the Najranites. Ali obeyed the orders and joined the Prophet only during the Farewell Pilgrimage. It was on the 25th of Thul-Qi’dah/February 25, 632 A.D. that the Prophet left Medina for hajj.

Note:
[1] al-Tabatabai, Al-Mizan fi Tafsir al-Qur'an, Vol. 3, pp. 269-270.