Why did God say of the People of the Book that they were given a portion of the Book, and what is the complete Book, and why did they not believe in the entire Book if they believed in Jesus without distortion?

In Arabic, "nasīb" (نصيب) means share or lot. The "min" (مِن) in the verse "nasīban min al-kitāb" (نصيبًا من الكتاب - a share of the Book) can be interpreted in two ways: either it signifies partiality, meaning they were given only a small portion of the Book, which would be a condemnation of them; or it is for explanation, meaning they were given a share, which is the Book itself. The Book referred to is the Torah, and the completeness of their faith in it necessitates their following of the Prophet (peace be upon him).