Allah states that the scholars of the People of the Scripture know the truth of what Allah's Messenger ﷺ was sent with, just as one of them knows his own child, which is a parable that the Arabs use to describe what is very apparent. Similarly, in a Hadith, Allah's Messenger ﷺ said to a man who had a youngster with him:
«ابْنُكَ هَذَا»
؟ قَالَ: نَعَمْ يَا رَسُولَ اللهِ أَشْهَدُ بِهِ
(Is this your son) He said, "Yes, O Messenger of Allah ﷺ! I testify to this fact." Allah's Messenger ﷺ said:
«أَمَا إِنَّهُ لَا يَجْنِي عَلَيْكَ وَلَا تَجْنِي عَلَيْه»
(Well, you would not transgress against him nor would he transgress against you.)
According to Al-Qurtubi, it was narrated that `Umar said to `Abdullah bin Salam (an Israelite scholar who became a Muslim), "Do you recognize Muhammad ﷺ as you recognize your own son" He replied, "Yes, and even more. The Honest One descended from heaven on the Honest One on the earth with his (i.e., Muhammad's) description and I recognized him, although I do not know anything about his mother's story."
Allah states next that although they had knowledge and certainty in the Prophet , they still:
لَيَكْتُمُونَ الْحَقَّ
(conceal the truth.)
The Ayah indicates that they hide the truth from the people, about the Prophet , that they find in their Books,
وَهُمْ يَعْلَمُونَ
(while they know it. ) Allah then strengthens the resolve of His Prophet and the believers and affirms that what the Prophet came with is the truth without doubt, saying:
الْحَقُّ مِن رَّبِّكَ فَلاَ تَكُونَنَّ مِنَ الْمُمْتَرِينَ
((This is) the truth from your Lord. So be you not one of those who doubt.)