Why does the believer need to fast?

God — who created everything for the humans that He might make of them living kings queens — gave the humans one commandment: not to eat of the tree of the knowledge of good and evil. He did not do so to deprive them of anything but wanted them to stretch out their hands and eat from the tree of life, and so to live, through obedience, in a blissful eternity, like the holy angels. Moreover, Adam and Eve could have resisted Satan and all his soldiers through obedience to the divine commandment, and thereby declare through their fasting that their love for God transcends everything. The Lord planted for them the Garden of Eden (Gen. 2:8) to make of their hearts an exalted paradise.

St. Basil the Great says, “Fasting began in Paradise when God said to Adam: ‘Do not eat from the tree of knowledge of the good and evil.’ Our first parents were driven out of Paradise for lack of fasting. Therefore, fasting is the way to enter the kingdom. Do not tremble or be afraid of fasting. Does your body not need medicine? Likewise, your soul also often needs medicine. Fasting is the medicine of the soul, to rid it of sin.”⁴¹²

Also, in the Old Testament deuterocanonical Book of Judith, the leaders and people trusted that the Lord would heed their prayers when they were steadfast in fasting and prayers in the sight of the Lord (Judith 4:8–15).