There are many themes twining through Rosh Hashanah. One important theme, as one might expect for the "birthday of the world" is creation. But the High Holidays Makhzor* reminds us:
"Creation, we are taught is not an act that happened once upon a time, once and for ever. The act of bringing the world into existence is a continuous process. God called the world into being, and that call goes on. There is this present moment because God is present. Every instant is an act of creation. A moment is not a terminal but a flash, a signal of Beginning. Time is a perpetual innovation, a synonym for continuous creation. Time is God's gift to the world of space."(page 329)The Makhzor also tells us:
"Moreover, our Sages taught, the human being is "God's partner in the work of Creation." He and we create together. There is still much work to be done: disease to be conquered, injustice and poverty to be overcome, hatred and war to be eliminated. There is truth to be discovered, beauty to be fashioned, freedom to be achieved, peace and righteousness to be established. There is a great need to dedicate all the creative power which a creating God has given us, so that we may join Him in 'the continuing work or Creation.'" (page 149)So Rosh Hashanah is a day to rededicate ourselves to being co-creators of the Universe, partners with G-d.
*Makhzor is a transliteration into English of the Hebrew word used for the book which contains the order of service, all the prayers, songs, poems (psalms), and teachings for Jewish services. The Makhzor used by my congregation, B'nai Sholom, is Makhzor Khadash/The New Makhzor for Rosh Hashanah and Yom Kippur, complied and edited by Rabbi Sidney Greenberg and Rabbi Jonathan D. Levine, The Prayer Book Press of Media Judaica, Inc. 1978.