“Four periods [of the year] have been set aside as times of abstinence, so that over the course of the year we might recognize that we are constantly in need of purification, and that amid life’s distractions, we should always strive by means of fasting and acts of charity to extirpate sin, sin which is multiplied in our transitory flesh and in our impure desires.” - Saint Leo the Great (5th Century)
“…the Nativity Forty-day Fast represents the fast undertaken by Moses, who, having fasted for forty days and forty nights, received the Commandments of God, written on stone tablets. And we, fasting for forty days, will reflect upon and receive from the Virgin of the living Word - not written upon stone, but born, incarnate, and we will commune of His Divine Body.” - Saint Symeon of Thessaloniki
The Church in Her wisdom has prescribed the Nativity fast as well as the three other great fasts to aid us as we develop as Christians. The term "prescribed" is often used as it implies that that there is something ailing us that needs healing or medication. The fast is one of the church's medicines for us. Also, since it is prescribed, it is in some sense, optional. Many participate at different levels, some observing that fast for all 40 days strictly, some less days and less strictly. Those new to the faith are often told not to "jump in" to the fast for there first year, but wade into the waters slowly. Some do not participate and remain the way they are with medicine always within reach. Though sometimes difficult and trying, though sometimes the medicine does not go down, it is always for our betterment.
Fasting has at its foundation, the denial of certain foods, but that is only the impetus for the true purpose of fasting which is the denial of self. If one was to keep the fast perfectly in regards to food, but does not love his neighbor, then he has not kept the fast at all. Fasting is to lead us contemplation about the season of the fast, in this case the birth of Christ, to more prayer, to more time reading the Scriptures, to more thankfulness, and to caring for the poor (what we call almsgiving). But even when doing those things, one has not truly kept the fast if it is not coupled with a denial of self. Choosing to bridle ones own tongue, to resist pride, to consider another more valuable than yourself, to pursue humility, to become Christlike.
Father Stephen on his post, "The Nativity Fast – Why We Fast" at his blog, "Glory to God for All Things" states:
Fasting is not dieting. Fasting is not about keeping a Christian version of kosher. Fasting is about hunger and humility (which is increased as we allow ourselves to become weak). Fasting is about allowing our heart to break.
He also tells an interesting story about fasting and death, which I will end this post with:
I worked for a couple of years as a hospice chaplain. During that time, daily sitting at the side of the beds of dying patients – I learned a little about how we die. It is a medical fact that many people become “anorexic” before death – that is – they cease to want food. Many times family and even doctors become concerned and force food on a patient who will not survive. Interestingly, it was found that patients who became anorexic had less pain than those who, having become anorexic, were forced to take food.
It is as though at death our bodies have a wisdom we have lacked for most of our lives. It knows that what it needs is not food – but something deeper. The soul seeks and hungers for the living God. The body and its pain become a distraction. And thus in God’s mercy the distraction is reduced.
Why do we fast? We fast so that we may live like a dying man – and in dying we can be born to eternal life.
An excellent podcast on fasting can be found at Ancient Faith Radio by Michael Hyatt called "At the Intersection of East and West". Micheal is the CEO of Thomas Nelson Bible Publishers as well as a deacon in the Orthodox Church.