A section of the People of the Book say: “Believe in the morning what is revealed to the believers, but reject it at the end of the day; perchance they may (themselves) Turn back;
“And believe no one unless he follows your religion.” Say: “True guidance is the Guidance of Allah: (Fear ye) Lest a revelation be sent to someone (else) Like unto that which was sent unto you? or that those (Receiving such revelation) should engage you in argument before your Lord?” Say: “All bounties are in the hand of Allah: He granteth them to whom He pleaseth: And Allah careth for all, and He knoweth all things.”
For His Mercy He specially chooseth whom He pleaseth; for Allah is the Lord of bounties unbounded.
Some from among the people of the Book chose a strategy of deception in order to mislead Muslims. According to their agreed plan, they would declare their belief in the Qur’an in the morning and then reject
it in the evening. They thought that this quick about-turn would
create doubts in the minds of Muslims about Qur’an and Islam and
they might turn away from their faith thinking that there must be
something wrong with Islam visible to such learned people who were
open-minded enough to accept it first. They also made it a point to reserve their feinted declaration of belief in Islam to hoodwink Muslims
only! Their true belief, they planned, would be affirmed before none
but only those who follow their own creed. The rule was: Sincerity
with the later, expediency with the former!
That these tricks are low is proved later in Verse 73 when the Holy prophet (S) has been asked to declare that their clever moves are not going to work for them since Guidance given to servants of AIlah is certainly from Allah. So, it is He who, with Guidance in His control, can make whoever He will steadfast on that Guidance. No one can
succeed in weaning such a person away from it with any trick, plan or enticement whatsoever.
The reason why they act in the way they do is envy, that is, they
envy Muslims as to why they were given the Scripture, or they envy
their supremacy in religious argument. It is because of this envy that they keep working to bring about the downfall of Islam and of the community that adheres to it. That this envy is based on not knowing the nature, timing and wisdom of Allah’s grace and mercy is explained in the later part of verse
73 and in verse 74.
Shafi, Muhammad (2008). Maariful Quran. (Vol .2 surah 3 pg. 96-97). English-MaarifulQuran-MuftiShafiUsmaniRA-Vol-2.pdf (islamicweblibrary.com)