Jesus’s instruction in verses 17 and 18 is almost comical in its plainness. Anoint your head. Wash your face. The modern translation is simply: look normal. Do what you do every morning. Take a shower. Get dressed. Go about your day. The contrast he is drawing is not between public and private in some abstract sense; it is between a person who deliberately signals sacrifice and a person who deliberately conceals it. Both are fasting. Only one is fasting toward God.
The logic here runs directly against what we might expect. We might assume that a deeper devotion would produce a more visible posture, that the more seriously we take our spiritual lives, the more our outward bearing would reflect it. But Jesus says the opposite. Genuine devotion produces a kind of hiddenness, a refusal to let the discipline become its own advertisement. Craig Blomberg notes that Jesus is not asking disciples to pretend they are not fasting; he is asking them to refuse the performance of it. There is a difference between concealing something and denying it. Jesus asks for the former, not the latter.
There is also something important in what Jesus does not say. He does not abolish fasting. He does not suggest it is outdated, unnecessary, or peculiar to Jewish piety. He says ‘when you fast,’ not ‘if you fast,’ placing it alongside giving and prayer as a discipline his followers will practice. The problem is never the discipline itself. The problem is always the audience. And the correction Jesus offers is not to eliminate the discipline but to reorient it, away from the crowd and back toward the Father who is in the secret place.