"she took her real text from St Matthew's Gospel"

In this verse in the Gospel of Matthew (10:16), Jesus is giving the disciples advice (or instruction) in their coming mission to spread the Word of God to the Jews and warns them of the dangers they face.  The disciples are the 'sheep' and, by implication, the Jews are the 'wolves' and hostile to the belief that Jesus is the son of God.  By referring to the wisdom of serpents, Jesus is recalling the serpent in the Garden of Eden in the Old Testament Book of Genesis where it is called 'subtle'.  But, as 'doves' they are to accomplish their mission with no harm to anyone else.

Matthew Evangelist. Koberger Bible 1483. Author Anton Koberger
Public DomainMatthew Evangelist. Koberger Bible 1483. Author Anton Koberger - Credit: Wikimedia Commons