Sa`id bin Jubayr, As-Suddi and others said that these Ayat were revealed concerning a delegation that An-Najashi (King of Ethiopia) sent to the Prophet in order to hear his words and observe his qualities. When the delegation met with the Prophet and he recited the Qur'an to them, they embraced Islam, cried and were humbled. Then they returned to An-Najashi and told him what happened. `Ata' bin Abi Rabah commented, "They were Ethiopians who embraced Islam when the Muslims who migrated to Ethiopia resided among them." Qatadah said, "They were some followers of the religion of `Isa, son of Maryam, who when they saw Muslims and heard the Qur'an, they became Muslims without hesitation." Ibn Jarir said that these Ayat were revealed concerning some people who fit this description, whether they were from Ethiopia or otherwise. Allah said,
Mentioned earlier was the friendliness of Jews with disbelievers. Mentioned now is their hostility towards Muslims in tandem with the disbelievers - which was the real cause of that friendship (the enemy of my enemy is my friend! ). And since the Qur'an upholds justice universally, it has not counted everyone even among Jews and Christians as being in the same lot. Whoever among them had some good quality, that was acknowledged openly - for example, the presence of a particular group among the Christians which did not have the kind of prejudice the Jews were known for, and the statement about the Christians who had responded heartily to the call of truth and for which they were considered worthy of the best of praise and reward. This particular group was that of the Christians of Ethiopia who did nothing which could cause problems for Muslims who had migrated to Ethiopia from their home in Makkah, before the Hijrah to Madinah. Any Christian, other than them, who is like them, shall also be legally counted as one of them. As for those who had accepted the truth, they are the King of Ethiopia, Najashi (Negus) and his courtiers who wept on hearing the Qur'an in their own country and embraced Islam. After that, a deputation of thirty men from there came to the Holy Prophet ﷺ who recited the Qur'an to them which they heard in tears and took their Shahadah as Muslims. This is the Cause or Back-ground of the revelation of this verse.
In this verse paradise is declared the reward for certain ‘words’ uttered by people (fa athabahumu’llah bima qalu). But what were these ‘words’ which made the speakers of them entitled to eternal paradise? What they said represented and gave expression to their entire being. It was the sound of the outflow of their personality. They heard God’s message in such a way that they found the whole truth enshrined in it. It penetrated their hearts and minds. It caused in them a revolution which changed the entire focus of their wishes and aspirations—to the extent that all barriers of prejudice and interests broke down. They identified themselves with Truth in such a way that they lost their separate identity. They became its witness and to become a witness meant truth embedding itself within them. The Quran was not a mere book for them but a living sign of the Lord of the Universe. This Divine experience, although expressed in words, was not just a matter of words: it was a turmoil, an earthquake which affected their entire existence. So much so that tears flowed from their eyes. Words spoken about reality do not merely make a statement: they are the highest form in which to give meaningful shape to a man’s actions and feelings, and, in the entire known universe, only man has the capacity to comprehend the refinement and significance of words and to use them to the best effect. ‘Words’ are the best of man’s being. ‘Word’ is eloquent action. Therefore, when a man proves and expresses his servitude to God at the level of ‘words’, he establishes a certain entitlement to paradise. The strongest reasons for non-acceptance of Truth by a man are his ego and pride. Those who have pride hidden in their hearts show the strongest reaction against the call for Truth, refusing to accept it at any cost and going so far as to become its most dangerous antagonists. On the other hand, those who are not filled with pride, while they may be flawed by any other evil, they cannot oppose the Truth to this extent.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.