well it seems that the forum ate my post, how annoying.
1. The Quran does not state what Christians actually believe
My answer:
We understand that the original followers of Christianity followed the true teachings of what was revealed to Jesus(as) It was after Jesus(as) ascended into heaven that the Christians began believing the falsehoods.. Please notice in your quote of the ayyat it begins: They say,
( a ) Does that mean Christians say or is it refering to those who call themselves Christian?
( b ) Keep in mind the majority of those who called themselves Christian were and still are Roman Catholic and Eastern Orthodox and that statement in the Quran is very much in line with Catholic belief and doctrine although the Catholic explanation is quite confusing as to how God begot a son equates to having a son .
( a ) even if we were to concede this point and term all trinitarians as pseudo-christians, it would still not change the fact that trinitarians don't at all claim that god took a son--that is adoptionism. trinitarians condemned adoptionism and as such the quoted surah has nothing to do with the trinity whatsoever and cannot at all function as a refutation thereof. i must however commend you on implicitly admitting that this concerns not mere semantics but that there is a very real theological difference between the two.
( b ) neither roman catholics nor eastern orthodox christians adhere to adoptionism and so i can't at all agree with that. they are trinitarian and condemn adoptionism as well. so therefore the statement in the qur'an is not at all in line with the position of these institutions because it fails to condemn the trinity (just as i had, in my "fristianity" example failed to condemn tawhid even though i did use the word "tawhid" or mentioned god in the singular. from the context of my post it was quite clear that i had an improper understanding of it and the same can be said for every reference of the trinity within the qur'an).
if i understand the section on "begot a son" and "having a son" then you acknowledge that the statement "god has a son" does not necessarily invoke the aspect of temporality while the phrase "the father begot the son" does seem to invoke this very same thing and as such they cannot be made to equate to one another (do let me know if i am mistaken in my understanding of what you meant). let us then move on to talk of this concept. in actuality, the phrase is not merely begotten but rather "only-begotten".
It is the Greek word “monogeneis.” This is not simply “begotten,” for that expression can be applied to all believers, those who have been begotten or born again by the Spirit. This is a unique expression for a unique person, the only-begotten Son of God. The expression appears in John 1:14, 1:18, 3:16, and 3:18. It would literally mean the “only generated one.” This is the key expression for the doctrine of “the eternal generation of the Son,” meaning, he always was the only begotten Son. The expression does not refer to the birth of Jesus in Bethlehem, because he is the Son from eternity past.
[...] You can only beget a child that has the same nature as you have—a son or a daughter. There is nothing else you can beget (unless you were speaking very figuratively). Your son or your daughter will inherit his or her nature from you—genes, personality—all of it. You can use “make” or “create” for producing a child; but when you use “beget” it only means you produce a child that has your nature.
Now follow this carefully. If Jesus is said to be the begotten Son of God (using the figure from human language to make the point), then Jesus has the same nature as the Father. If Jesus has the same nature as God the Father, then Jesus is divine and eternal as well. If he is eternally God, then there was never a time he was literally begotten—which is why we know the language is figurative to describe his nature, and not his beginning. To call Jesus “the only begotten Son” means that he is fully divine and eternal. He is God the Son.
from the above it becomes obvious that trinitarians don't at all mean to imply a sexual union between god and a woman (as it is misunderstood in the qur'an for how then would the speaker almost derisively ask, "how can allah have a son when he has no wife?") for that would be blasphemy. instead the term "only-begotten" is merely to signify the relationship which the son has to the father. that said, it then becomes plainly evident that the phrase "god begot a son" is not in contradiction to "god has a son" and that neither of these involve temporality.
2. You believe it is an ancient heresy that God took a Son
My answer
I agree it was and it still is a heresy believed by many who call themselves Christian
we indeed are in agreement but once more that is not what trinitarians believe. at most that verse would condemn adoptionism and trintarians themselves condemn this. it therefore stands that there is no repudiation of the trinity at all within the qur'an seeing as whenever it engages in a description of the three gods whom christians worship, it is never that which trinitarians actually profess to worship. mary always makes an appearance as one of the three and seeing as we know that the trinity deals with three persons and given that the only enumeration of three supposed deities we have are allah, mary, and jesus it follows that the qur'an has an improper understanding of this trinity which trinitarians hold so dear.
3. God always was 3 entities inseperable from each other
My answer
This is our point of biggest disagreement. By simple definition in my feeble, somewhat senile mind. if they are inseparable they are not 3 entities. If they are not 3 entities, there is no trinity
if i have understood your example correctly then you are arguing that distinctions cannot exist when things are inseparable (please correct me if i am wrong). at present, it is our task to see if this argument is at all correct. you would agree with me that length is distinct from, width, and that width is distinct from height, and that height is not either of these and yet it is not 3 spaces that we possess but rather that the one space is always existent as length, width and height. these are distinct (such that neither is the other), yet are all the one space (such that prerogatives of space apply equally and fully to these three distinctions), and therefore these aspects of space are inseparable as it relates to space. simply from this, we can therefore admit that 3 things can be distinct without losing that which makes them inseparable.
4. Christians have always understood the doctrine of the trinity correctly.
My answer
Then why did it take so many church councils to define it and work it into Church Doctrine?
no, the argument of mine which you number as the fourth does not say that christians as a whole always understood the trinity (i am not ready to call someone who does not understand the trinity not a christian) but rather that even if we were to grant that at some point in the distant past mary was part of the trinity (which i don't believe but only grant for the sake of the argument), by the time of muhammad (actually a very long time before he began spreading the message of islam), the trinity had been defined as the father, the son, and the holy spirit. as such, the qur'an would have no reason to consistently condemn misrepresentations of the trinity while supposing that it was actually condemning the thing that trinitarians believed. what is condemned is almost every heresy from adoptionism, sabellianism, tritheism, (and a heresy which i don't even know the name of: confounding the position of the father in S. 5:70-75. the father wouldn't even be the third.) yet never the trinity.
anyway, i will await your reply.
— vb , -gets , -getting , -got , -gat , -gotten , -got 
