And there are, certainly, among the People of the Book, those who believe in Allah, in the revelation to you, and in the revelation to them, bowing in humility to Allah: They will not sell the Signs of Allah for a miserable gain! For them is a reward with their Lord, and Allah is swift in account.
Appreciating the good people among them As in the above verses and in the sūrah as a whole, the People of the Book have been severely criticised for their conduct and attitude. Now, as the sūrah closes, the sincere ones among them, who faithfully followed the teachings of their scriptures and later embraced Islam, are praised for their efforts and sacrifices. In this, there is an extremely subtle suggestion that the good they had in their ranks have by now been separated and absorbed in the community of Islam; and that whoever is left behind is nothing more than buttermilk with its cream removed. All such people will find their special reward waiting for them with their Sustainer. They are also assured that their promised reward will be given to them without any delay, even before their sweat has dried. This is precisely the message the words innallāha sarī‘u-l ḥisāb (Allah is swift at reckoning) carry when used, as here, in a context of hope and comfort. Allah is very quick in settling an account!
Islahi, Amin Ahsan. Pondering Over The Qur’an: Surah Ali Imran (p. 330). Islamic Book Trust. Kindle Edition.