and they maintain that Sophia, herself has also spoken many things through them regarding the first Anthropos (man), and concerning that Christ who is above, thus admonishing and reminding men of the incorruptible light, the first Anthropos, and of the descent of Christ. The [other] powers being terrified by these things, and marvelling at the novelty of those things which were announced by the prophets, Prunicus brought it about by means of Ialdabaoth (who knew not what he did), that emissions of two men took place, the one from the barren Elizabeth, and the other from the Virgin Mary.
12. And since she herself had no rest either in heaven or on earth, she invoked her mother to assist her in her distress. Upon this, her mother, the first woman, was moved with compassion towards her daughter, on her repentance, and begged from the first man that Christ should be sent to her assistance, who, being sent forth, descended to his sister, and to the besprinkling of light. When he recognised her (that is, the Sophia below), her brother descended to her, and announced his advent through means of John, and prepared the baptism of repentance, and adopted Jesus beforehand, in order that on Christ descending he might find a pure vessel, and that by the son of that Ialdabaoth the woman might be announced by Christ. They further declare that he descended through the seven heavens, having assumed the likeness of their sons, and gradually emptied them of their power. For they maintain that the whole besprinkling of light rushed to him, and that Christ, descending to this world, first clothed his sister Sophia [with it], and that then both exulted in the mutual refreshment they felt in each other's society: this scene they describe as relating to bridegroom and bride. But Jesus, inasmuch as he was begotten of the Virgin through the agency of God, was wiser, purer, and more righteous than all other men: Christ united to Sophia descended into him, and thus Jesus Christ was produced.
12. And since she herself had no rest either in heaven or on earth, she invoked her mother to assist her in her distress. Upon this, her mother, the first woman, was moved with compassion towards her daughter, on her repentance, and begged from the first man that Christ should be sent to her assistance, who, being sent forth, descended to his sister, and to the besprinkling of light. When he recognised her (that is, the Sophia below), her brother descended to her, and announced his advent through means of John, and prepared the baptism of repentance, and adopted Jesus beforehand, in order that on Christ descending he might find a pure vessel, and that by the son of that Ialdabaoth the woman might be announced by Christ. They further declare that he descended through the seven heavens, having assumed the likeness of their sons, and gradually emptied them of their power. For they maintain that the whole besprinkling of light rushed to him, and that Christ, descending to this world, first clothed his sister Sophia [with it], and that then both exulted in the mutual refreshment they felt in each other's society: this scene they describe as relating to bridegroom and bride. But Jesus, inasmuch as he was begotten of the Virgin through the agency of God, was wiser, purer, and more righteous than all other men: Christ united to Sophia descended into him, and thus Jesus Christ was produced.
(Irenaeus, Against Heresies, 1.30.11-12)
If I understand well this passage, some points are rather evident:
1) there is separationism between the ''son of Ialdabaoth'' (probably Sabaoth, aka the mere man Jesus) and the spiritual Christ who descend on him.
2) the role of the Baptist is to make this ''son of Ialdabaoth'' a “pure vessell” (in advance for the spiritual Christ) by an apt “baptism of repentance”.
So why did the Gnostics used just John the Baptist to vehicle this theology?
For two reasons:
1) the legend said that John was a Baptizer.
2) the legend was rather obscure about who was really John.
If Greg Doudna is right to claim that Josephus ''euhemerized'' the legend of “John the Baptizer” (= he explained it on the table by inventing ex nihilo a figure of a John killed by Herod Antipas, when really the true John was Hyrcanus II), then the two conditions above were satisfied. Any person on the earth couldn't claim a full knowledge of John the Baptist so he was the perfect Mr. Unknown who could introduce in the world the Unknown par excellence, Jesus Christ.
ADDENDA:
I find rather suggestive when the author writes:
They further declare that he descended through the seven heavens, having assumed the likeness of their sons, and gradually emptied them of their power.
This fits the idea that the Christ, by possessing Jesus (that is: having assumed the likeness of Jesus), was really empyting him of the his power, him who is the son of Ialdabaoth (the evil demiurge).