How does Allah (the Exalted) permit the marriage of a Muslim man to a woman of the People of the Book (Christian or Jewish), despite His saying (the Exalted): "They have certainly disbelieved who say that Allah is the third of three"?

The ruling on marrying Kitabiyah (women from the People of the Book) went through three stages:

The first stage: From the beginning of the call to Islam until the Treaty of Hudaybiyyah (6 AH). During this period, Muslims were allowed to maintain their marriages, including mixed marriages between Muslims and non-Muslims, due to the nascent stage of the call and the limited authority of Muslims.

The second stage: Began in the year 6 AH and extended until the Farewell Pilgrimage (10 AH). During this stage, marriage to any female disbeliever or polytheist was forbidden, including Kitabiyah, based on the Almighty's saying: (and do not hold on to the bonds of disbelieving women) and (And do not marry polytheistic women until they believe). This was to firmly establish the severing of allegiance between Muslims and the people of disbelief.

The third stage: Began during the Farewell Pilgrimage (10 AH). In this stage, marriage to chaste women from the People of the Book was permitted, based on the Almighty's saying: (and chaste women from those who were given the Scripture before you). However, the prohibition of marrying other disbelieving women remained. This was permitted after the rulings on severing allegiance between Muslims and non-Muslims were well-established, and the harm to a Muslim's faith became secure, coupled with the presence of da'wah (preaching) interests aimed at attracting Kitabiyah women and their relatives to Islam.