‘Umar Ibn Al-Khaṭṭab asked Ka‘b Al-Aḥbar -a Jewish scholar who converted to Islam-: “had you known him (the Prophet (saws)) Ka‘b? Ka‘b answered: I had known him as I had known my son, yet I had known Muhammad better”. The People of the Holly Books know the Prophet (saws), because he is mentioned in the books of the children of Israel, as well as they mentioned his descriptions and the descriptions of his nation.
(وَإِنَّ فَرِيقًا مِّنْهُمْ لَيَكْتُمُونَ الْحَقَّ وَهُمْ يَعْلَمُونَ) (a party of them conceal the truth while they know [it]: According to the (probability mmaintenance law), (a party of them): some of them said the truth, as evidence Ka‘b Al-Aḥbar had become a Muslim. Thus, a party of them conceal the truth, and not all of them.