And Your word is true and endures forever. (Rosh Hashana Prayers) Although this phrase, which concludes the blessing of the sanctification of the day, is said only on Rosh Hashanah, it does not appear to have any specific connection to the festival. Perhaps, however, a well-known Midrash (Bereishis Rabbah 8:5) about the creation of the universe can shed some light on its relevance to this day:
Said Truth, “Do not create a world because it will be all falsehood,” and peace said, “do not create because it will be all conflict.”
In spite of the protests raised by Truth and Peace, Hashem did create the universe, and each on the anniversary of that creation Israel is judged anew on its performance in upholding these qualities. In His great kindness, however, Hashem assists us in meeting the exacting standards which Truth and Peace demand for the universe to continue to exist.
Actually, truth is one of the main themes of Rosh Hashanah. As we have explained in length elsewhere, the shofar we blow on Rosh Hashanah reminds of the giving of the Torah, the ultimate truth. Moreover, The wordless shofar represents the pure unadulterated sound which always remains truthful, even if words are often used to mislead.
Therefore, in concluding the Sanctification of the Day we remind Hashem of Torah – His “word [which] is true and established forever.”