I was looking for the term perfidious as I found a link that says this was the term used to describe Jews in the pre-Vatican-II liturgy for Good Friday. I wonder why it was used to describe Jews in such a way.
In the Good Friday liturgy prior to 1959, the prayer for the conversion of the Jewish people read:
For the conversion of the perfidious Jews. Let us pray also for the Jews that the Lord our God may take the veil from their hearts and that they also may acknowledge our Lord Jesus Christ.
Let us pray: Almighty and everlasting God, you do not refuse your mercy even to the Jews; hear the prayers which we offer for the blindness of that people so that they may acknowledge the light of your truth, which is Christ, and be delivered from their darkness.
The term perfidious was removed in 1959 by John XXIII, but the rest of the prayer remained the same. In 1969, the prayer was changed to:
Let us pray for the Jewish people, the first to hear the word of God, that they may continue to grow in the love of his name and in faithfulness to his covenant.
Almighty and eternal God, long ago you gave your promise to Abraham and his posterity. Listen to your Church as we pray that the people you first made your own may arrive at the fullness of redemption.
One of the definitions of the word perfidious is “faithless” (i.e., without faith). In the context of the prayer quoted above, a distinction was being drawn between those Jews who had accepted Jesus of Nazareth as the Messiah and those who had not by labeling the latter group “perfidious.” Unfortunately, that particular word was open to serious misunderstanding because it can also mean “treacherous” and “deceitful” and thus can be considered insulting to Jews who in good conscience cannot believe that Jesus of Nazareth is the Messiah for whom they await.
Later, the entire prayer was reworked to make the petition for the conversion of the Jews a positive supplication that both honored the faith and truth that the Jewish people have and have held for thousands of years, and asked for them to receive the fullness of that faith and truth that is found in Christ Jesus.