Next came the practice of ritual vows, soon to be followed by religious pledges and sacred oaths. Most of these oaths were accompanied by self-torture and self-mutilation; later on, by fasting and prayer.  Fasting and other forms of self-denial were thought to be pleasing to the ghosts, who took pleasure in the discomfort of the living. 
Sacrifice as a part of religious devotions, like many other worshipful rituals, did not have a simple and single origin. As civilization advanced, these crude concepts of sacrifice were elevated to the level of the rituals of self-abnegation, asceticism, fasting, deprivation, and the later Christian doctrine of sanctification through sorrow, suffering, and the mortification of the flesh. 
For Isaiah, acceptable fasting was not afflicting the soul, bowing the head, or dressing in sackloth. What he asked was to loosen the bonds of wickedness, undo the knots of heavy burdens, let the oppressed go free, break every yoke, share bread with the hungry, bring home the homeless poor. 
Fasting is not part of gospel. Nazarites refused to accept Jesus as a teacher sent from heaven because he did not teach fasting and other forms of self-denial. Jesus said: “In the old order you fasted and prayed; as the new creature of the rebirth of the spirit, you are taught to believe and rejoice”. 
It is most dangerous to knowingly engage in spiritual fasting in order to improve one’s appetite for spiritual endowments. Prolonged fasting, either physical or spiritual, tends to destroy hunger.