Matthew 6:16 "And when you fast, do not look gloomy like the hypocrites, for they disfigure their faces that their fasting may be seen by others. Truly, I say to you, they have received their reward. 17 But when you fast, anoint your head and wash your face, 18 that your fasting may not be seen by others but by your
Father who is in secret. And your Father who sees in secret will reward you."
Fasting is humbling ourselves before God to show we are grieved over sin. It is humbling ourselves before God to be heard by God. Joel 2:12-14 is an example of fasting, "Yet even now," declares the LORD, "return to me with all your heart, with fasting, with weeping, and with mourning; 13 and rend your hearts and not your garments." Return to the LORD, your God, for he is gracious and merciful, slow to anger, and abounding in steadfast love; and he relents over disaster. 14 Who knows whether he will not turn and relent, and leave a blessing behind him, a grain offering and a drink offering for the LORD
your God?" Notice that fasting is to be an external act to reveal a condition of the heart. Fasting is a way to show our genuine dependence upon God and sorrow for sin. In 2 Chronicles 7:14 Solomon instructs God's people what to do when they become at odds with the Lord because of their sin, "if my people who are called by my name humble themselves, and pray and seek my face and turn from their wicked ways, then I will hear from heaven and will forgive their sin and heal their land."
We are to fast to be seen by God and not by man. The Jewish leaders of Jesus day fasted to show people how righteous they were. There fasting was an external show to impress people and not a matter of their heart. Jesus revealed their internal condition in Matthew 23:27-28 "Woe to you, scribes and Pharisees, hypocrites! For you are like whitewashed tombs, which outwardly appear beautiful, but within are full of dead people's bones and all uncleanness. 28 So you also outwardly appear righteous to others, but within you are full of hypocrisy and lawlessness."
God resists the proud, but gives grace to the humble (1 Peter 5:5b). When we fast we are to do it from our heart to be seen by God and not to impress people. God will see we are genuinely grieved over sin and desire to gain His help to overcome sin and be restored to wholeness. When we fast, we are not to make a big production of it, but go on living a normal life just as if we were not fasting. God will see and reward us.