Verse 5:5 permits marriage with people who have been " given the book before you ":
والمحصنات من المؤمنات والمحصنات من الذين أوتوا الكتاب من قبلكم
And [lawful in marriage are] chaste women from
among the believers and chaste women from among those who were given
the Scripture before you ...
— Quran 5:5
And it is understood by the majority of the scholars that it includes Christians: trinitatians or otherwise.
This is because the Christians had similar beliefs in the time of the Prophet Muhammad ﷺ and the Quran itself mentioned that Christians consider Jesus to be divine [5:72], and that they believe in trinity [5:73] and having that in mind it still did not exclude them in 5:5.
A group has opposed the above, because of the following verse:
And do not marry mushrikati (polytheistic women) until they
believe. And a believing slave woman is better than a polytheist, even
though she might please you. And do not marry mushrikeena ( polytheistic men) [to
your women] until they believe. And a believing slave is better than a
polytheist, even though he might please you. Those invite [you] to the
Fire, but Allah invites to Paradise and to forgiveness, by His
permission. And He makes clear His verses to the people that perhaps
they may remember.
— Quran 2:221
They claim that 2:221 abrogates 5:5 and cited the view of Ibn Umar:
Whenever Ibn `Umar was asked about marrying a
Christian lady or a Jewess, he would say: "Allah has made it unlawful
for the believers to marry ladies who ascribe partners in worship to
Allah, and I do not know of a greater thing, as regards to ascribing
partners in worship, etc. to Allah, than that a lady should say that
Jesus is her Lord although he is just one of Allah's slaves."
— Bukhari
The majority however disagrees and their arguments include:
2:221 is only about polytheist idolators and does not extend to Christians, since the Quran normally uses mushriqeen to only refer to the idolaters and the Christians are usually separately mentioned as Nasara or as Ahle Kitab (people of the book) ... for example consider:
إِنَّ الَّذِينَ آمَنُوا وَالَّذِينَ هَادُوا وَالصَّابِئِينَ
وَالنَّصَارَىٰ وَالْمَجُوسَ وَالَّذِينَ أَشْرَكُوا إِنَّ
اللَّهَ يَفْصِلُ بَيْنَهُمْ يَوْمَ الْقِيَامَةِ ۚ إِنَّ اللَّهَ عَلَىٰ
كُلِّ شَيْءٍ شَهِيدٌ
Indeed, those who have believed and those who were Jews and the
Sabeans and the Christians and the Magians and
those who associated with Allah - Allah will judge between them on the Day of Resurrection. Indeed Allah is, over all things, Witness.
— Quran 22:17
مَّا يَوَدُّ الَّذِينَ كَفَرُوا مِنْ أَهْلِ الْكِتَابِ وَلَا
الْمُشْرِكِينَ أَن يُنَزَّلَ عَلَيْكُم مِّنْ خَيْرٍ مِّن
رَّبِّكُمْ ۗ وَاللَّهُ يَخْتَصُّ بِرَحْمَتِهِ مَن يَشَاءُ ۚ وَاللَّهُ
ذُو الْفَضْلِ الْعَظِيمِ
Neither those who disbelieve from the People of the Scripture nor
the polytheists wish that any good should be sent down to you from
your Lord. But Allah selects for His mercy whom He wills, and Allah is
the possessor of great bounty.
— Quran 2:105
لَمْ يَكُنِ الَّذِينَ كَفَرُوا مِنْ أَهْلِ الْكِتَابِ
وَالْمُشْرِكِينَ مُنفَكِّينَ حَتَّىٰ تَأْتِيَهُمُ الْبَيِّنَةُ
Those who disbelieved among the People of the Scripture and the
polytheists were not to be parted [from misbelief] until there came to them clear evidence -
— Quran 98:1
The scope of the words is different. ahl al-kitab is more specific than mushrikeen which is general, so 5:5 makes an exception in the general ruling of 2:221. The complete command is to not marry polytheists except people of the book. This is an example of تخصيص in عموم and there are several other examples of this.
5:5 abrogated 2:221 it in their regard, since al-Maidah was revealed after al-Baqarah.
Marriage with people of the book is cited from some of the Sahaba including the caliph Uthman ibn Affan who married Na'ila bint al-Furafisa while she was a Christian.
There is a detailed discussion of this and the arguments and counter-arguments in:
Tafsir Al Jassas on 2:221 and 5:5
Tafsir Al Qurtubi on 2:221 and 5:5
