The church continued writing its experience of God after the death of the Apostles. I certainly admit and teach that that post-apostolic literature is not Scripture. It is not inspired. The writers were not infallible, and, therefore, their writings were not inerrant. They were, however, helpful in establishing the understanding that they had of the teachings of the Apostles.
One of those texts is an anonymous epistle called the Didache, or "The Teachings of the Twelve." It was written approximately in 97AD, so, soon after the death of John, the last surviving Apostle. We find this in 7:1-3: "Concerning Baptism. And concerning baptism, baptize this way:
Having first said all these things, baptize into the name of the Father, and of
the Son, and of the Holy Spirit, in living water. But if you have no living
water, baptize into other water; and if you cannot do so in cold water, do so in
warm. But if you have neither, pour out water three times upon the head into the
name of Father and Son and Holy Spirit."
My point in bringing this up is that it addresses the claims of Oneness Pentecostals that Trinitarian baptism was invented by the Council of Nicea, and did not exist before that time. In addition to denying the authenticity of the baptismal formula in Matthew 28:19, they claim that their view is supported by the baptisms in Acts, which are in some variation of "in the name of Jesus."
The above passage demonstrates two important things: first, the doctrine of the Trinity was taught from the beginning, not created later; and second, that the church used the Trinitarian baptismal formula from the beginning, as well. It proves that the Oneness historical claims are false, inventions for the sake of defending their doctrine, not supported by any evidence. Even their claim regarding Matthew 28:19 has zero textual support, since all of the manuscripts (at least, so far discovered) contain the traditional text: "Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit." The Oneness doctrine is the invention of men, not of the Holy Spirit. When I have brought this passage to the attention of Oneness believers, their only response was, "It's a lie!" That is a convenient response to evidence against a pet doctrine.