There is no contradiction here: the Quran condemns false beliefs found among Christians and Jews, yet still gives them a distinct legal category as "People of the Book." A ruling can recognize that a belief is wrong while still treating its followers differently from pagans with no revealed scripture.
Why the question feels forceful
The argument is not silly. The Quran says:
"Do not marry polytheistic women until they believe."
Quran 2:221
But it also says:
"This day all good foods have been made lawful for you. The food of those who were given the Scripture is lawful for you and your food is lawful for them. And lawful for you are chaste women from among the believers and chaste women from among those who were given the Scripture before you."
Quran 5:5
Then the Quran strongly criticizes Christian doctrines:
"They have certainly disbelieved who say, 'Allah is the Messiah, the son of Mary'... They have certainly disbelieved who say, 'Allah is the third of three.'"
Quran 5:72-73
So the challenge is: if such beliefs are shirk, why are Christian women not included in the ban on marrying mushrik women?
The Quran uses more than one legal category
The answer is that "shirk" can describe a theological error, while "mushrik" in a legal ruling may refer to a particular legal class. The Quran itself distinguishes between "those who associate partners" and "the People of the Book" in many places, even while condemning corrupt beliefs among some of the People of the Book.
Classical scholars explained the marriage ruling in two main ways, both leading to the same result. Some said Quran 5:5 specifies and limits the general wording of Quran 2:221: polytheist women are forbidden, but chaste Jewish and Christian women are made an exception by a later, explicit text. Others said the word "mushrikat" in Quran 2:221 refers primarily to pagan idolaters, not to the People of the Book, because the Quran commonly treats them as separate communities in law.
Either way, the legal result is clear: a chaste woman from the People of the Book is not treated like an idol-worshipping pagan woman for this specific ruling, even if her community holds doctrines the Quran rejects as disbelief or association.
A distinction is not an approval
This permission does not mean the Quran approves of the Trinity, divine sonship, or taking religious authorities as lords besides Allah. It plainly rejects those beliefs. The point is narrower: the People of the Book retain a special status because they are linked to earlier revelation, prophets, scripture, and a moral-religious tradition, even though their beliefs contain serious corruption.
That is why the same chapter can criticize Christian theology and still allow marriage to chaste Christian women. A law may say, "This group is mistaken in creed," and also say, "They are not legally identical to pagan idolaters in every ruling."
What the ruling proves - and what it does not
It proves that Islamic law has layered categories. Not every form of disbelief is treated the same in every practical ruling. Atheists, pagan idolaters, and People of the Book are all outside Islam, but the law does not always deal with them identically.
It does not prove that Christian doctrine is acceptable, or that all interfaith marriages are wise, easy, or encouraged in every circumstance. The permission is conditional: chastity is mentioned, and the Muslim husband remains responsible for protecting his religion and the religious welfare of his household. The Quran also makes clear that Muslim women are not given in marriage to unbelieving men:
"They are not lawful wives for them, nor are they lawful husbands for them."
Quran 60:10
So the strongest answer is simple: the Quran condemns shirk wherever it appears, including in corrupted beliefs among the People of the Book, but it still assigns the People of the Book a distinct legal status. That distinction is not a loophole or contradiction; it is part of the Quran's own legal structure.