This Wikipedia article concerning Gog and Magog says Rashi wrote that the Christians would ally with Gog and Magog to destroy Israel, only to be thwarted in the end. Where does he write this?
According to the book that the article links to (https://books.google.com/books?id=AVMzAQAAQBAJ&pg=PA54&hl=en#v=onepage&q&f=false), in Isaiah 18:4, Rashi says that the text refers to the victory over Esau. Rashi writes
"מתשלום שכרו של גוג"
And sites that translate the Rashi, such as the Chabad site read,
"from paying Esau his just desserts"
So Gog becomes Esau. In the linked book (2 pages earlier) there is extensive discussion of how Rashi connects Esau to Christianity.
Additionally, there is this statement
In a number of places, Rashi identifies Esav and Edom with Rome (that is, the Romans). The Jews of Europe in the Middle Ages would identify Rome with Christianity and the (Roman) Catholic Church; therefore, prophecies in the Bible which speak about Esav and Edom were understood by them as relating to Christianity, as were the references of the Sages to Esav, Edom, or Rome. See G.D. Cohen, “Esau as Symbol in Early Medieval Thought,” Alexander Altmann (ed.), Jewish Medieval and Renaissance Studies (Cambridge, 1967). See also the course offered by the Open University of Israel, “Bein Yehudim Le-Notzerim: Yehudim Ve-Notzerim Be-Ma’arav Europa ad Reishit Ha-Et Ha-Chadasha,” Prof. Ora Limor (1993-1997), Vol. I, pp. 9-15 in particular.
Put all together, there is a connection between the verse, the idea and the reference.
Rashi does not explicitly say that it is the Christians. Wikipedia points to Grossman, Avraham (2012). "The Commentary of Rashi on Isaiah and the Jewish-Christian Debate". In Wolfson, Elliot R.; Schiffman, Lawrence H.; Engel, David. Studies in Medieval Jewish Intellectual and Social History. Brill. p. 54. ISBN 9789004222366.
He refers to Rahsi's comment on Yeshayahu 18:4
For so has the Lord said to me, "I will rest, and I will look down upon My dwelling-place, like a clear heat upon herbs, like a cloud of dew in the heat of harvest.
I will rest: from paying Esau his just deserts; I will turn away from all My affairs and I will look down upon My dwelling-place to do good to it.
He then says
Knowing Rashi's association of the Christians as allies of Gog and Magog, they will all be thwarted as punishment for their planned obliteration of yhe Israelites.
It is apparent that he assumes that since Esav and Edom are connected with the Roman Empire and with the Christians that followed them, then the reference to Esav in Rashi also refers to the Christians.
It is clear that Rashi's identification of the Christians as allies of Gog and Magog attests to the intensity of his hatred of Christian domination of medieval Europe.
Thus, this is an assumption of the author of this document rather than an explicit statement by Rashi.