When we fast and abstain during the holy season of Lent, we do so in order to open up our minds and bodies to the more regular and habitual presence of Christ in our lives. Lent is a time when we ask the Lord to give us grace to use such abstinence, that, our flesh being subdued to the Spirit (Coll: Lent I) we may obey Him always, follow the movements of his will and embrace his love. During Lent we put aside diversions, distractions and occupations that tend to deplete our spiritual energy and concentration. In it we pray that holy desire and longing for God’s presence increase within us.
Fasting is an exercise in cleansing. Before holy desire or spiritual longing can grow, we must be emptied of the many vices and bad habits that define our lives. St. Augustine puts it this way: The exercise of fasting will be effective only to the extent that we free ourselves from the desires leading to infatuation with this world. Take the example of filling an empty container. God means to fill each of you with what is good; so cast out what is bad! If he wishes to fill you with honey and you are full of sour wine, where is the honey to go? The vessel must be emptied of its contents and then be cleansed. (Serm. 1st John: Augustine)
The problem is that, by nature, we do not tend to want to rely on anyone or anything outside of ourselves. We do not want to be emptied in order to be then filled. The Credo of post-Christian America seems to be that we can make it on our own, by our efforts, pursuing our own desires. We are so full or ourselves that we ignore the need for self-emptying and God-filling. Self-fulfillment and self-maintenance seem to prevail in people’s lives. But this is of course is delusional and fatal to the intentions that God has for us. As C.S. Lewis says, God made us, he invented us as a man invents an engine. A car is made to run on gasoline, and it would not run properly on anything else. Now God designed the human machine to run on Himself. He Himself is the fuel our spirits were designed to burn, or the food our spirits were designed to feed on. There is no other. (Mere Christianity)
We need God because he is fuel and energy that makes the human machine run at its best. We cannot be filled by the Holy Spirit until we know that relying on our own energy means that we are running on empty. That is the point of the fast.
St. Michael and All Angels Sermons