Ooops,meant to post the full link
Here it is
http://www.newadvent.org/cathen/15047a.htm
You may wish to read St. John of Damascus' 8th century Exact Exposition of the Orthodox Faith, Book I.
Trinitarian dogma is a revealed truth. Using the NT as "proof" is difficult not only because there is only indirect evidence to support it, but because we have such latter-day fraudulent insertions as the Comma Johanneum.
The fact is that we really have no way of knowing what was in the original Gospel accounts. St. Mark's Gospel, for example, is much shorter in the oldest extant copies.
We really have only shreds of older copies, but even they are only copies of copies and not originals. So, when we see the Great Commission we have no way of knowing if it was inserted at a latter-date (like the Comma Johanneun), to make the Bible fit the dogma or whether it was there from the beginning.
Certainly, even St. Paul doesn't speak in trinitarian terms, nor does he think much of Baptism which is the apex of the Gospel's trinitarian formula, yet his Epistles are the oldest NT writings and therefore the closest to the original church mindset.