Gnostics
Some of the things gnostics say sound promising to 'Oneness' Pentecostals:
"Now the name of the Father is the Son. It is he who first gave a name to the one who came forth from him, who was himself, and he begot him as a son. He gave him his name which belonged to him; he is the one to whom belongs all that exists around him, the Father, His is the name; his is the Son." (The Gospel of Truth, The Nag Hammadi Library, James M. Robinson, p. 49)
Sabellius
Because Sabellius is the most famed of the modalists, Christians on first encountering 'Oneness' Pentecostals debate against Sabellian tenets remembered from theology text-books. The response they hear is invariably a shocked, 'But we don't believe anything like that!' Indeed, Sabellius' vocabulary, of Father "dilating" into Son and Holy Spirit, is not commonly heard from the 'Oneness' crowd:
"Sabellius also raves in saying that the Father is Son, and again, the Son Father, in subsistence One, in name Two; and he raves also in using as an example the grace of the Spirit. For he says, 'As there are "diversities of gifts, but the same Spirit," so also the Father is the same, but is dilated into Son and Spirit.'" (Athanasius, Four Discourses Against the Arians, Discourse 4, 25).
"If then the Monad being dilated became a Triad, and the Monad was the Father, and the Triad is Father, Son, and Holy Ghost, first the Monad being dilated, underwent an affection and became what it was not; for it was dilated, whereas it had not been dilate. Next, if the Monad itself was dilated into a Triad, and that, Father and Son and Holy Ghost, then Father and Son and Spirit prove the same, as Sabellius held, unless the Monad which he speaks of is something besides the Father, and then he ought not to speak of dilation, since the Monad was to make Three, so that there was a Monad, and then Father, Son, and Spirit." (Athanasius, Four Discourses Against the Arians, Discourse 4, 13).
Will the real 'Oneness' Pentecostals please stand up?