Why does Mtt 27:19 mention the post- dream message of Pilate's wife?

Since the Gospel of Matthew is addressed primarily to Jewish Christians, it reflects Jewish traditions in which dreams are recognized as a legitimate form of divine revelation, as often seen in the Old Testament. Matthew records six dreams (1:20-25; 2:12; 2:13-14; 2:19-21; 2:22; and 27:19), whereas no dreams appear in Mark, Luke or John. Matthew's inclusion of the dream given to Pontius Pilate's wife likely serves to underscore Jesus' innocence, confirmed through divine revelation.

The gospel of Luke, composed after Matthew, does not include this dream. Unlike Matthew, Luke writes mainly for a Gentile audience, for whom dreams carried different cultural and religious connotations. Luke also emphasizes Jesus' innocence, but he does so through narrative detail rather than revelatory dreams. He provides an extended account of Jesus' trial before Pilate (Luke 23:1-25), highlighting that Pilate attempts three times to release Jesus. Luke 23:22 records Pilate's final attempt: “Why? What crime has this man committed? I have found in him no grounds for the death penalty. Therefore I will have him punished and then release him.”'

Observation

A central Christian belief is that salvation comes through the redemptive death of Jesus, a sinless man who offers himself for sinful humanity. Matthew highlights Jesus' innocence through divine revelation - the dream of Pilate's wife. Luke highlights the same truth through Pilate's repeated reluctance to condemn Jesus. Their differing approaches appear to reflect the cultural expectations of their respective audiences: Matthew writes for Jews familiar with revelatory dreams, while Luke writes for Gentiles who would respond more readily to legal and narrative demonstrations of innocence.

To the main question (why does Matthew mention this episode?) the only thing that can be said for certain is that Matthew, alone among the gospel writers, had heard the story (unless he made it up). So I would say that, since he knew of it, there was no good reason for him not to include it.

Other questions:

Did the Evangelist want to drive his readers home to the immense public pressure that Pilate was under to condemn Jesus?

Possibly. This attitude is mentioned by Matthew Henry, for example, who says: "The Jews were so bent upon the death of Christ, that Pilate thought it would be dangerous to refuse."

Or, was it intended to show how hardcore Pilate was that in spite of getting warned, he went ahead to appease Jewish leaders?

I would answer no to this. Rather, Matthew portrays Pilate as being ready to let Barabbas suffer instead of Jesus. Shortly after mentioning the dream of Pilate's wife, Matthew relates:

24 When Pilate saw that he was not succeeding at all, but that a riot was breaking out instead, he took water and washed his hands in the sight of the crowd, saying, “I am innocent of this man’s blood. Look to it yourselves.”

How do scholars explain the relevance of Mt. 27:19 mentioning the post-dream message of Pilate's wife?

Responses fall into four main groups:

1. The dream came from the devil in order to frustrate God's plan, which was to sacrifice Jesus on the Cross. Ellicott and Calvin mention this possibility, but do not endorse it. Gill provides the counter argument:

Some have thought, that this dream was from the devil, willing to hinder the death of Christ, and so man's redemption and salvation by it; but had he had any such intention, the most effectual method would have been to have persuaded the chief priests and elders off of it...

2. The dream came from God in order to emphasize Jesus' innocence. Calvin is one example:

We ought to conclude that God the Father took many methods of attesting the innocence of Christ, that it might evidently appear that he suffered death in the room of others, — that is, in our room. God intended that Pilate should so frequently acquit him with his own mouth before condemning him, that in his undeserved condemnation the true satisfaction for our sins might be the more brightly displayed.

3. The story of the dream was a legend that Matthew had heard. Meyers does not agree with this, but he mentions several scholars who do.

There is nothing to show that Matthew intended us to regard this incident as a special divine interposition. There is the less reason for relegating it to the domain of legend (Strauss, Ewald, Scholten, Volkmar, Keim). {df - This is also the view of more recent scholars such as Crossan and Spong}

Conclusion: Although there is no consensus, the majority view is that Matthew relates the dream of Pilate's wife to emphasize Jesus' innocence.

Pilate's response to his wife's warning about the nightmare she had, is shown in Matthew's account. Commentator J.C. Ryle explains:

"Though willing to save our Lord's life, [Pilate] was afraid to do it if it offended the Jews... [After] washing his hands publicly before the people, he at last condemned One whom he himself called a 'just person'! He rejected the strange and mysterious warning which his wife sent to him after her dream: he stifled the remonstrances of his own conscience. He 'delivered Jesus to be crucified'." Expository thoughts on Matthew's Gospel", J.C.Ryle, p.386 (Banner of Truth 1986 emphasis mine.)

He ignored his wife's warning. Apparently, he said not a word to her (certainly nothing that is recorded in scripture), and she would not have been standing before him as she "sent him a message". But silence speaks volumes. Especially if it is deliberately part of ignoring a person. That was his response to his wife.

Second (speculative) thought from a different source:

"Romans often viewed dreams as omens. The dream was probably given by God as a sign of Jesus' innocence." E.S.V. footnote

Third view, which is a shrewd observation about the wife, and the irony of it all:

"It is ironic that a pagan recognizes and takes a stand for Jesus' innocence, while the Jewish crowd does not." N.L.T. footnote

All quite interesting, especially if taken together.