And yet, the Didache was written much earlier than that, as early as AD 50, and while it says nothing of Matt 28:19, it does say to baptize in the name of the Father, the Son, and the Holy Spirit.In the only codices which would be even likely to preserve an older reading, namely the Sinaitic Syriac and the oldest Latin Manuscript, the pages are GONE which contained the end of Matthew 28. Frederick Cornwallis Conybeare (1856 - 9 January 1924) Professor of Theology at the University of Oxford.
Here is the oldest recorded document of Matthew 28:19.
"The Demonstratio Evangelica" by Eusebius:
Eusebius of Caesarea. 265 ? AD.– 337 ? AD.
Eusebius was the Church historian and Bishop of Caesarea. On page 152 Eusebius quotes the early book of Matthew that he had in his library in Caesarea. Eusebius informs us of Yahshua's actual words to his disciples in the original text of Matthew 28:19.
Quote: "With one word and voice He said to His disciples: "Go, and make disciples of all nations in My Name, teaching them to observe all...
And again Eusebius for example, in Book III of his History, Chapter 5, Section 2, which is about the Jewish persecution of early Christians, we read:
"But the rest of the disciples, who had been incessantly plotted against with a view to their destruction, and had been driven out of the land of Judea, went to all nations to preach the good news, relying upon the power of Christ, who had said to them, "Go ye and make disciples of all the nations in my name."
And again, in his Oration in Praise of Emperor Constantine, Chapter 16, Section 8, we read:
"What king or prince in any age of the world, what philosopher, legislator or prophet, in civilized or barbarous lands, has attained so great a height of excellence, I say not after death, but while living still, and full of mighty power, as to fill the ears and tongues of all mankind with the praises of his name?
Surely none save our only Savior has done this, when, after his victory over death, he spoke these words to his followers, and fulfilled it by that event, saying to them, "Go ye and make disciples of all nations in my name."
There is not a single occurrence of the disciples baptizing anyone using the Trinitarian formula. All of the scripture in the New Testament shows that people were baptized into the name of Yahshua, even after Pentecost.
Justin Martyr (c. 100 - c. 165), in First Apology, chapter 61, says, "For, in the name of God, the Father and Lord of the universe, and of our Saviour Jesus Christ, and of the Holy Spirit, they then receive the washing with water." While he isn't quoting Matt 28:19, it is in full agreement. You'll also notice how early the doctrine of the Trinity was being believed and written about.
Iranaeus says in Against Heresies (c. 180), book 3, chapter 17, section 1: 'And again, giving to the disciples the power of regeneration into God, He said to them,” Go and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost.”'
Tertullian also mentions the triadic formula in On Baptism (AD 200-206), chapter XIII: ' For the law of baptizing has been imposed, and the formula prescribed: “Go,” He saith, “teach the nations, baptizing them into the name of the Father, and of the Son, and of the Holy Spirit.”' In Prescription Against Heretics, chapter 20, he says, 'He commanded the eleven others, on His departure to the Father, to “go and teach all nations, who were to be baptized into the Father, and into the Son, and into the Holy Ghost.”'
There is plenty of early evidence that the church was using the triadic formula and were commanded to do so, whether from Matt 28:19 or otherwise. That Matt 28:19 was changed is questionable.
Regardless, it’s just one verse out of many, and it isn’t needed to support the doctrine of the Trinity.