It's already been demonstrated Ad Nazorean here that GMark has parallels to Paul as to themes. One of Paul's major themes is to contrast the supposed physical with the supposed spiritual for the same subject.
1 Corinthians 12
13 For in one Spirit were we all baptized into one body, whether Jews or Greeks, whether bond or free; and were all made to drink of one Spirit.
The offending word:
1 Corinthians 12:13
907 [e] ebaptisthēmen ἐβαπτίσθημεν, were baptized, V-AIP-1P
The false prophet Paul's supposed point is that the physical baptism (water) does not make someone righteous. It is the spiritual baptism (spirit) that does. The theme is baptism and the contrast is physical baptism verses spiritual baptism.
"Mark" (author) gives narrative birth to the theme. Of specific interest here is that "Mark" invokes the brave and influential prophet from The Jewish Bible, Isaiah, by name twice, in order to connect Paul's baptism theme in GMark:
2 Even as it is written in Isaiah the prophet, Behold, I send my messenger before thy face, Who shall prepare thy way.
3 The voice of one crying in the wilderness, Make ye ready the way of the Lord, Make his paths straight;
4 John came, who baptized in the wilderness and preached the baptism of repentance unto remission of sins.
5 And there went out unto him all the country of Judaea, and all they of Jerusalem; And they were baptized of him in the river Jordan, confessing their sins.
1 And there are gathered together unto him the Pharisees, and certain of the scribes, who had come from Jerusalem,
2 and had seen that some of his disciples ate their bread with defiled, that is, unwashen, hands.
3 For the Pharisees, and all the Jews, except they wash their hands diligently, eat not, holding the tradition of the elders;
4 and [when they come] from the market-place, except they bathe themselves, they eat not; and many other things there are, which they have received to hold, washings of cups, and pots, and brasen vessels.)
5 And the Pharisees and the scribes ask him, Why walk not thy disciples according to the tradition of the elders, but eat their bread with defiled hands?
6 And he said unto them, Well did Isaiah prophesy of you hypocrites, as it is written, This people honoreth me with their lips, But their heart is far from me.
So note that in all of GMark there are only two invocations of "Isaiah". Both involve supposed prophecy regarding baptism (7:4 has the same offending word). "Mark's" supposed point is that the physical baptism does not make anyone righteous. The first invocation of Isaiah in Chapter 1 shows all "The Jews" receiving a physical baptism. The subsequent narrative though shows that no one becomes righteous based on that. Jesus is the only one who has a physical baptism who becomes righteous but the text is clear that the cause
is the "baptism" by the spirit. The second invocation of Isaiah at Chapter 7 has "Mark's" Jesus make the point explicit. All "The Jews" have physical (water) ritual baptism but that does not make them righteous. The further implication is that it actually causes unrighteousness because it makes "The Jews" think they are righteous. It is the spiritual baptism of Jesus' teaching (revelation) that makes one righteous.
Of course the irony here (and one that I think the author of "Mark" would have really appreciated) is that (the) holy spirit failed to tell Paul that this teaching of the supposed danger of physically washing your hands would end up costing the lives of millions of Believers during the Black Plague and even worse that the survivors would falsely accuse and murder untold numbers of Jews of blessed memory for holding fast to the command of God and washing before they ate.