Teshuvah
Thursday night, the month of Elul begins. The day specifically mentioned in the Torah as the day of atonement is the tenth of Tishrei, Yom Kippur, but the entire ten days from Rosh HaShana until Yom Kippur are also called the "Ten days of repentance". And to a lesser extent the entire month of Elul is devoted to repentance as well. For this reason this month has special customs such as blowing the shofar each morning and for Sefaradim saying selichot each night or morning.
This pattern was established by Moshe Rabbeinu, who climbed Mount Sinai at the beginning of Elul to seek forgive- ness for the sin of the Golden Calf, and finally descended forty days later on 10 Tishrei with new tablets of the law, proving that God had forgiven us. (See Rashi on Devarim 9:18.)
The literal meaning of the Hebrew word for repentance, "teshuva", is "return", and the prophets have revealed that repentance brings us back as it were, to God Himself (Malachi 3:7). This is the ultimate return, to return to our source in the Source of holiness. So it seems strange that the tenth of Tishrei, Yom Kippur, the day that Moshe succeeded in bringing complete forgiveness to the Jewish people, was the day that he descended from the mountain, from G-d's presence.
It seems that the ultimate goal of our repentance is not to completely withdraw from this world into an attenuated spiritual existence, but rather to take advantage of our return to holiness to then go back and infuse this holiness into the world.
It is true that anyone who wants to approach holiness needs to withdraw to a certain extent from involvement in the world and its pleasures - like Moshe Rabbenu, who went without eating and drinking for forty days and nights. This is the importance of the various customs of asceticism that begin in Elul and increase as we approach Yom Kippur, when we eschew all the main material comforts.
But exactly when we achieve the highest levels of sanctity and insight, we are called to descend the mountain, to show how sanctity and insight can be applied and experienced without completely withdrawing from the main areas of human activity and aspiration.
Just as Moshe brought us the Torah on Yom Kippur, we use our spiritual ascent to strengthen our ability to live Torah lives - not to climb by drawing away from the world, but to sanctify the world through performance of mitzvot and hence draw the world upwards with us.