The Canterbury Cathedral is related to Thomas a Becket because Becket was murdered there. Henry II made Becket his chancellor and the two became close friends. When the position of Archbishop became vacant, Henry II made Thomas a Becket the Archbishop of Canterbury.
Henry II wanted to reduce the power of the Church and established the Constitutions of Clarendon which allowed the King to punish a cleric found guilty by the Church. Becket refused to sign the Constitutions of Clarendon and was found guilty of treason. He fled to France and was only allowed to return when the Pope threatened to excommunicate Henry II. Upon his return, Becket excommunicated three bishops who supported Henry. This enraged Henry and he ordered his knights to kill Becket.
On December 29, 1170, Becket was murdered at the altar of Canterbury Cathedral by four knights of King Henry II.
Its job is to attract the readers attention and its explained with the help of supporting details
I would say the appeals George Washington used was pathos and ethos, considering those were the ones that appealed best to the audience's emotions.