The Roman governor Turnus Rufus asked Rabbi Akiva, "Whose deeds are better- G-D's or human being's?" Rabbi Akiva replied, "A human being's deeds are better!" Turnus Rufus then asked him, "Why are you circumcised?" Rabbi Akiva replied by bringing Turnus Rufus bundles of raw wheat and fresh baked hot rolls from Angel's Bakery.
Rabbi Akiva then said to him, "Which are tastier-G-D's raw wheat or our cheese Danishes?" This proves says Rabbi Akiva, that our work is better than G-D's works. Turnus Rufus replied to him, "If G-D desires circumcision, then why doesn’t the baby leave the womb already circumcised?" Rabbi Akiva Answered, "Because G-D gave Israel the Torah in order for us to improve the world through the Mitzvot." (Midrash Tanchuma on Parashat Tazria).
Rabbi Akiva compared the circumcision of a newborn baby to the baking of bread. Just as raw wheat is not a complete product, but we have to transform it into edible bread, so too, a person is born incomplete and it's up to him to complete himself.
How could it be that our actions are better than those of G-D? Why doesn't G-d create the world complete, rather than leaving so much work for us to accomplish? The Roman governor believed that Creation is perfect, and man can not perfect himself or his environment. Brit Mila was seen by him as desecrating and damaging the human body.
Rabbi Akiva proved to him that it is man who turns the raw wheat into delicious bread. Using this example, Rabbi Akiva was teaching that we must get involved with the realty around us by perfecting ourselves. Even the Roman governor can not eat raw wheat for breakfast!
Brit Mila is a symbolic act that brings us into the cycle of responsibility, to work towards TIKKUN OLAM.
The Sefer HaChinuch on the Mitzvah of Mila explains it this way, "And G-D wanted completion of the world to be done by us, and so He did not create man complete from birth, to teach him that when he completes the shape of his body by his own hand through Mila, so by his own hand he can also complete the shape of his soul through his deeds."
Our mission is to improve and perfect the world by improving and perfecting ourselves.