3:199
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 swiftin account.
Although the preceding verses—and the sūrah as a whole—contain strong criticism of the conduct and attitudes of many among the People of the Book, the concluding passage acknowledges and praises those among them who remained sincere. These individuals faithfully adhered to the teachings of their scriptures and subsequently accepted Islam, and they are commended for their commitment and sacrifices. The passage subtly indicates that the virtuous elements within the People of the Book had by this stage aligned themselves with the Muslim community, leaving behind those who did not follow the path of sincerity.
Such individuals are promised a distinct reward from their Lord in recognition of their faith and righteousness. The text assures them that their recompense will not be delayed and will be granted promptly. This meaning is conveyed in the phrase innallāha sarīʿu al-ḥisāb (“Indeed, Allah is swift in reckoning”), which in this context functions as a message of reassurance and hope, emphasizing the immediacy and certainty of divine reward for those who act with sincerity and devotion.
References:
Islahi, Amin Ahsan. Pondering Over The Qur’an: Surah Ali Imran (p. 330). Islamic Book Trust.Kindle Edition.