I remember reading a statement that the third paragraph of the Shema was changed to a different paragraph from the Torah that we now use. The reasoning I read was that it was a measure against the Christians who no longer wore tzitzit nor followed most of God's commandments. I'm looking to find if this is based in fact or not.
Are you thinking of the Shortened Third Paragraph said in Israel?
אמר רב יוסף כמה מעליא הא שמעתתא דכי אתא רב שמואל בר יהודה אמר אמרי במערבא ערבית דבר אל בני ישראל ואמרת אליהם אני ה' אלהיכם אמת
Rav Yosef said: How excellent is this tradition that I heard, as when Rav Shmuel bar Yehuda came from Eretz Yisrael to Babylonia he said: In Eretz Yisrael, at the evening prayer they do not recite the entire third portion of Shema, which deals with ritual fringes, as there is no obligation to wear ritual fringes at night. Rather, they say a condensed version of that portion that includes an excerpt from the beginning and an excerpt from the end: “Speak to the children of Israel and say to them…I am the Lord, your God, True” (Numbers 15:38, 41).
Source - Berachos 14b.