Adoptionism at Jesus' baptism is in keeping with Epiphanius' comments that the Matthew of one branch of Jewish Christians did not have the first two chapters and that he doesn't know if the Matthew used by another branch had "the genealogies from Abraham to Christ" or if they believed in the virgin birth.
Pan. 30.13.2-7:
Now in what they call a Gospel according to Matthew, though it is not the entire Gospel but is corrupt and mutilated—and they call this thing 'Hebrew'!—the following passage is found ... But the beginning of their Gospel is, 'It came to pass in the days of Herod, king of Judea, in the high-priesthood of Caiaphas, that a certain man, John by name, came baptizing with the baptism of repentance in the river Jordan, and he was said to be of the lineage of Aaron the priest, the son of Zacharias and Elizabeth, and all went out unto him.'
And after saying a good deal it adds, 'When the people had been baptized Jesus came also and was baptized of John. And as he came up out of the water the heavens were opened, and he saw the Holy Spirit in the form of a dove which descended and entered into him. And (there came) a voice from heaven saying, Thou art my beloved Son, in thee I am well pleased, and again, This day have I begotten thee.
Pan. 29.7.6:
As to Christ, I cannot say whether they .. regard him as a mere man—or whether, as the truth is, they affirm that he was born of Mary by the Holy Spirit.
Pan. 29.9.4:
They have the Gospel according to Matthew in its entirety in Hebrew. For it is clear that they still preserve this as it was originally written, in the Hebrew alphabet.But I do not know whether they have also excised the genealogies from Abraham till Christ.
And Irenaeus, who is the first church father to mention Jewish Christians, says they did not believe in the virgin birth.
AH 3.21.1:
God, then, was made man, and the Lord did Himself save us, giving us the token of the Virgin ... The Ebionites ... assert that He was begotten by Joseph; thus destroying, as far as in them lies, such a marvellous dispensation of God, and setting aside the testimony of the prophets which proceeded from God.
AH 5.1.3:
Vain also are the Ebionites, who do not receive by faith into their soul the union of God and man, but who remain in the old leaven of [the natural] birth, and who do not choose to understand that the Holy Ghost came upon Mary, and the power of the Most High did overshadow her
But other church fathers (Origen, Eusebius, Jerome) say that there was a branch of Jewish Christians who believed in the virgin birth, and I want to take a fresh look at them. Eusebius says:
EH 3.27.1-3:
The ancients quite properly called these men Ebionites, because they held poor and mean opinions concerning Christ.
For they considered him a plain and common man, who was justified only because of his superior virtue, and who was the fruit of the intercourse of a man with Mary. In their opinion the observance of the ceremonial law was altogether necessary, on the ground that they could not be saved by faith in Christ alone and by a corresponding life.
There were others, however, besides them, that were of the same name, but avoided the strange and absurd beliefs of the former, and did not deny that the Lord was born of a virgin and of the Holy Spirit. But nevertheless, inasmuch as they also refused to acknowledge that he pre-existed, being God, Word, and Wisdom, they turned aside into the impiety of the former, especially when they, like them, endeavored to observe strictly the bodily worship of the law.
And Origen says:
Against Celsus 5.61:
Let it be admitted, moreover, that there are some who accept Jesus, and who boast on that account of being Christians, and yet would regulate their lives, like the Jewish multitude, in accordance with the Jewish law—and these are the twofold sect of Ebionites, who either acknowledge with us that Jesus was born of a virgin, or deny this, and maintain that He was begotten like other human beings.
And Jerome says:
Epist. 112.4.13:
In our own day there exists a sect among the Jews throughout all the synagogues of the East, which is called the sect of the Minei, and is even now condemned by the Pharisees. The adherents to this sect are known commonly as Nazarenes; they believe in Christ the Son of God, born of the Virgin Mary; and they say that He who suffered under Pontius Pilate and rose again, is the same as the one in whom we believe. But while they desire to be both Jews and Christians, they are neither the one nor the other.
But before their time (and just after Irenaeus), Hippoltyus appears to be saying that there were Jewish Christians who believed in the virgin birth but still maintained that Jesus "received Christ" at his baptism:
RH 7.22-23:
The Ebionaeans, however, acknowledge that the world was made by Him Who is in reality God, but they propound legends concerning the Christ similarly with Cerinthus and Carpocrates. They live conformably to the customs of the Jews, alleging that they are justified. according to the law, and saying that Jesus was justified by fulfilling the law. And therefore it was, (according to the Ebionaeans,) that (the Saviour) was named (the) Christ of God and Jesus, since not one of the rest (of mankind) had observed completely the law. For if even any other had fulfilled the commandments (contained) in the law, he would have been that Christ. And the (Ebionaeans allege) that they themselves also, when in like manner they fulfil (the law), are able to become Christs; for they assert that our Lord Himself was a man in a like sense with all (the rest of the human family).
But there was a certain Theodotus, appropriating, however, (his notions of) Christ from the school of the Gnostics, and of Cerinthus and Ebion, he alleges that (our Lord) appeared in some such manner as I shall now describe. (According to this, Theodotus maintains) that Jesus was a (mere) man, born of a virgin, according to the counsel of the Father, and that after he had lived promiscuously with all men, and had become pre-eminently religious, he subsequently at his baptism in Jordan received Christ, who came from above and descended (upon him) in form of a dove. And this was the reason, (according to Theodotus,) why (miraculous) powers did not operate within him prior to the manifestation in him of that Spirit which descended, (and) which proclaims him to be the Christ.
So it looks to me like even the Jewish Christians who believed in the virgin birth maintained that Jesus "received Christ" at his baptism.