Quote from Evangelion:
How do you account for its appearance in the Didache and the various texts from early Church Fathers such as Iganatius and Cyprian (who quotes it pefectly, along with the entire verse as we have it today)?
An excellent question, Evangelion! And one that requires an answer from anyone tending to believe that the so-called "Trinitarian formula" of baptism found in Matthew 28 was added to the text of Matthew at a later date.
The earliest post-biblical Christian writers were the apostle Paul's fellow-labourer, Clement [A.D. 30-100] and his contemporary Ignatius [A.D. 30-107]. As you are probably aware, there is extant a shorter and a longer recension of the main writings purporting to be those of Ignatius. The quote "in the name of the Father, Son, and Holy Spirit" is found only in the longer recension. Most experts consider the shorter one as more likely
to be what Ignatius actually wrote. Other experts believe that we do not possess
any genuine writings of Ignatius today.
The "writings of Ignatius" which we possess today, uphold the elder-overseer distinction, a distinction which was not made by Paul, Peter, or any of the other apostles. In his letters to the churches, the writer recommended regarding that the believers regard the overseer (or "bishop") as they would regard Jesus Christ Himself. This does not seem characteristic of the first-century apostolic practice.
Third-century Cyprian lived in a day when Trinintarian thinking was common in the church, although the dogma was not universally accepted.
His contemporary, Novatian, wrote an article about the Trinity.
The didache certainly gives evidence of having been written at a very early date. Yet, some of the advice given therein seems to be ritualistic ---- again not characteristic of age. Perhaps portions of the didache were also interpolated by later writers. Some of these later writers were unscrupulous in their practice of adding their ideas to Christian texts in order to make them appear to have validity.
Tertullian [A.D. 145-200] was at some point a Montanist for a period of time. This sect was later declared a "heresy" by the main church. Tertullian held to a sort of proto-type of the Trinitarianism that later developed. He was the first known to use the word "Trinity" in a writing. Indeed, it may have been he who initially copied the "Trinitarian Formula" into Matthew 28.