Oneness Pentecostals teach that Jesus was the Father before His incarnation. They explicitly deny the preexistence of the Son. However, they hold their doctrine by resolutely shutting their minds to the communications between Jesus and the father.
For example, during the crucifixion, we see these words of Jesus: "
Father, into Your hands I commit My spirit" (Luke 23:46). Oneness teachers claim that this is the Son (which they define as the flesh nature of Jesus only), addressing the Father, that is, His own deity. They suppose, in other words, that this is the verbalization of the internal conversation between the human and divine natures of Christ.
But is that a logical conclusion from the grammar of the sentence? No, it's not.
First, would a rational person address Himself in the second person, "You," and claim that he is doing something to himself? No, he wouldn't. Second, if a person is speaking to himself, does he speak verbally, with the expectation that others would take note of his internal conversation? Again, no, he wouldn't. Third, would the Evangelist record those words as a conversation, be unaware that it was just Jesus speaking to Himself? Surely not! Yet, Luke records the words in the same way he does any other conversation between two people. Oneness believers assume that they are clever enough to figure out such a fundamental truth, even though Luke gives no hint of it in the text (or anywhere else, or by any other writer).
On the other hand, if we read Luke as natural language, written by a rational person about another rational person, and intended for an audience of rational persons, the grammar of the sentence points naturally to one person's speaking to a second, distinct person. That is, taking the text as natural language, rather than as a mysterious code for an exalted audience (a thoroughly-Gnostic concept), we can never come to the conclusion that Oneness teachers do. That can only indicate that their interpretation has a source other than the text.