The messages in stained glass are multi-faceted. Here we see some examples from York Minster, England. The stained glass draws the visitor's attention in multiple ways.
1) sheer size: The Great East Window behind the altar is exactly that -- great. It is the largest medieval stained glass window and although many of the panels have been replaced over time, they are still faithful to the original 1400s design depicting the beginning and end of the world.
2) familiar stories: The story from the Book of Genesis in the Great East Window is an example of a story that would have been familiar to the medieval church-goers. Adam and Eve eating the forbidden fruit followed by their ejection from the Garden Eden serve as both a warning against sin and reassurance of God's power.
3) comical yet moralizing stories: The scene of the ape's funeral procession is an example of how stained glass could turn the funny, even satirical, into a moral story. The placement of this window in the nave is at low height, making it readily readable for the average medieval and modern visitor.