Muslims tend to see this the Trinity as a union between God and
Mary giving birth to Jesus.
The Christian will be puzzled
by such an allegation because the Christian doctrine of Trinity
has in the Godhead God the Father, God the Son (Jesus) and God
the Holy Spirit. Many researchers believed that either Muhammad
was wrong, or that it was refering to some heretical sects.
D.A. Rice writes in an internet newsgroup posting :
[...rest deleted...]
I just want to remind readers of this thread to consider another
possible interpretation, mentioned by Jamal-al-din Beaven (Jim Beaven)
and myself. This is that there is evidence that an early sect of
Christianity considered Mary as part of the Trinity. Please see his
post and my post in this thread for more details.
In summary, parts of a document called the "Gospel of the Hebrews" has
been quoted by the Church Fathers Origen and Jerome, writing in the
3rd and 4th centuries CE. This document explicitly identifies Mary
with the Holy Spirit. This strongly suggests that the Christian sect
which accepted this early Gospel believed that Mary was divine, and
part of a Trinity.
It is possible that this sect still existed in the early 6th century
CE, when the Qur'an was revealed. Therefore, it seems likely to me
that Qur'an
al-Ma'idah 5:119
(which says that Jesus (a.s.) will deny teaching
that he and Mary were divine) is addressed specifically to this early
Christian sect which probably considered Mary to be divine, and any
other possible sects with a similar belief.
Contemporary Christians in general do not consider Mary to be divine.
The Qur'an declares the error of the Trinity as believed in by many
contemporary Christians in other verses.