He Described Fasting as Already Happening
When Jesus taught about fasting, he assumed his listeners were already doing it — not commanding them to start.
In Matthew 6:16–18, Jesus uses the second-person future indicative construction 'when thou fastest' (hotan nēsteuēs) — not 'if thou fastest' or 'you should fast.' The conditional is temporal, not hypothetical. This mirrors the same grammatical structure he uses in the surrounding instructions about almsgiving ('when thou doest thine alms,' 6:2) and prayer ('when thou prayest,' 6:5). In all three cases, Jesus does not introduce the practice — he assumes it is already underway and corrects how it is being performed. The issue he addresses is not whether his hearers fast but the manner of their fasting: the 'disfigured faces' and the 'sad countenance' designed to be seen by others. His instruction is entirely about concealment — 'anoint thine head, and wash thy face' — not initiation. Uniquely among the three Sermon practices (alms, prayer, fasting), fasting receives no positive promise except the generic 'thy Father which seeth in secret shall reward thee openly.' Yet the structure of all three is identical: assumed practice, condemned manner, commanded concealment, promised reward. What the red letters reveal is that Jesus treated fasting as a social institution already in operation, requiring correction, not introduction — a posture that is invisible unless you read his exact conditional phrasing.
Red Letter Sources
Matthew 6:2Matthew 6:5Matthew 6:16Matthew 6:17Matthew 6:18
Therefore when thou doest thine alms, do not sound a trumpet before thee, as the hypocrites do in the synagogues and in the streets, that they may have glory of men. Verily I say unto you, They have their reward.
And when thou prayest, thou shalt not be as the hypocrites are: for they love to pray standing in the synagogues and in the corners of the streets, that they may be seen of men. Verily I say unto you, They have their reward.
Moreover when ye fast, be not, as the hypocrites, of a sad countenance: for they disfigure their faces, that they may appear unto men to fast. Verily I say unto you, They have their reward.
But thou, when thou fastest, anoint thine head, and wash thy face;
That thou appear not unto men to fast, but unto thy Father which is in secret: and thy Father, which seeth in secret, shall reward thee openly .
Evidence Notes
All three Sermon on the Mount practices (alms, prayer, fasting) use the temporal 'when' construction, not a hypothetical 'if.' The Greek hotan with subjunctive in each case frames the action as anticipated, not optional. Fasting specifically is addressed at 6:16–18 with corrective rather than initiatory language.
Greek Note
Greek hotan (ὅταν) + subjunctive = 'whenever' or 'when,' indicating an action assumed to occur, not merely possible. Contrasts with ei + indicative, which would carry a genuinely conditional sense. The same hotan construction appears at 6:2 (alms) and 6:5 (prayer), making the structural parallel across all three disciplines explicit in the Greek.