Matthew 6:16-18,  advice on fasting

Hypocritical fasting

6:16 “Moreover when you fast, don’t be like the hypocrites, with sad faces. For they disfigure their faces, that they may be seen by men to be fasting. Most certainly I tell you, they have received their reward.
6:17 But you, when you fast, anoint your head, and wash your face;
6:18 so that you are not seen by men to be fasting, but by your Father who is in secret, and your Father, who sees in secret, will reward you.”

(Matt 6:16-18 WEB)

In tackling the topic of fasting Jesus addresses the third of Judaism’s so-called Three Pillars of piety. As when he tackled the previous two, his focus remains upon the hypocritical false witness that men can so easily substituted for a true relationship with God.

The Bible cites various instances of people fasting and throughout the history of the church there have been many who have fasted and subsequently affirmed it is a valuable spiritual discipline. Yet, it is one thing to tell people about your experience of fasting retrospectively, it is another to go out of your way to make sure that men see that you are fasting. It is those who do the latter whom Jesus accuses of hypocrisy. By their sad demeanour they want people to understand that they are fasting and to applaud them for the intensity of their spiritual life. If that is what they desire, says Jesus, then they their reward will be the attention that they receive from men, and they need not expect anything further by way of reward from God. Their manifest appearance of spirituality thus belies the truth about their impoverished relationship with God.

As with alms giving and prayer, Jesus affirms the practice (‘when you fast’ not ‘if you fast’), but instructs that it be done surreptitiously, in a manner that will deliberately avoid any human reward, that the reward may come from the God who sees.