Hence it makes a lot of sense if the Baptism Episode and the equation “John the Baptist = Elijah” was introduced only later in the Gospels, only after that Elijah appeared only during the Transfiguration episode, in a position of clear inferiority in comparison to Jesus.
Therefore John the Baptist was co-opted by the Christians only to make him the new Elijah, and thereby who precedes Jesus along the lines of a continuity, and not of a striking discontinuity, between “old” and “new”, between Law and Gospel.
This talks in support of Marcionite priority, since the baptism was absent in the Gospel of the heresiarch.
This explanation is more probable than the traditional explanation.
Since the traditional explanation requires that Jesus explained that John is Elijah to his disciples:
11 And they asked him, “Why do the teachers of the law say that Elijah must come first?”
12 Jesus replied, “To be sure, Elijah does come first, and restores all things. Why then is it written that the Son of Man must suffer much and be rejected? 13 But I tell you, Elijah has come, and they have done to him everything they wished, just as it is written about him.”
12 Jesus replied, “To be sure, Elijah does come first, and restores all things. Why then is it written that the Son of Man must suffer much and be rejected? 13 But I tell you, Elijah has come, and they have done to him everything they wished, just as it is written about him.”
(Mark 9:11-13)
...contra factum that even a blind realizes easily from the incipit of Mark that John is Elijah redivivus (hence the Jesus's explanation to disciples that “Elijah is already came” is entirely not-necessary and redundant).
At contrary, it is decisively more expected that Mark 9:11-13 was an interpolation to reiterate insistently again and again that the Transfiguration happened after the Baptism, and not before.
But the excessive emphasis betrayes the traces of the Judaizing interpolator.