14 “For if you forgive men their trespasses, your heavenly Father will also forgive you.
15 But if you do not forgive men their trespasses, neither will your Father forgive your trespasses.
16 “Moreover, when you fast, do not be like the hypocrites, with a sad countenance. For they disfigure their faces that they may appear to men to be fasting. Assuredly, I say to you, they have their reward.
17 But you, when you fast, anoint your head and wash your face,
18 so that you do not appear to men to be fasting, but to your Father who is in the secret place;and your Father who sees in secret will reward you openly.
When you fast (v.16) — a reference both to fasting prescribed under the Mosaic law in connection with the Day of Atonement (Leviticus 16:29) and the voluntary fast of that day. The Pharisees added two fast days, on Monday and Thursday of each week, as a case of public display and piety. The true purpose of fasting was intended, however, for deep contrition and spiritual communion. Fasting was especially emphasized as an effective means of dealing with temptation (cf. Isaiah 58:6). The Pharisees regarded the practice of fasting as meritorious and appeared in the synagogues negligently attired. Their sad disfigurement of face and the wearing of mourning garb gave them an opportunity to exhibit their superior ascetic sanctity before the people. — King James Bible Commentary, page 1185
The Pharisees fasted every Monday and Thursday; and since it happened opportunely that these were the days when the synagogue met, it was given them to display themselves to the assembled worshipers in their guise of woe. Their fasting was not merely abstinence from meat and drink. They did not wash or anoint themselves, they went barefoot, and they sprinkled ashes on their heads, “making their faces unsightly that they might be a sight to men in their fasting.” — Pentecost, page 184.
anoint your head and wash your face (v.17) — look as you usually do when you go out
Not surprisingly, my commentaries have little to say about verses 14 and 15 because they so clearly don’t fit in with our concept of forgiveness under grace but they don’t wish to make this distinction. There is a passing mention that the forgiveness here is not the forgiveness connected with salvation, but refers to the comfort God gives those who are penitent. Of course, that distinction wasn’t made by the Lord in this sermon because His audience was still under law and His words meant just what they said. Once again, as through this entire sermon, He was explaining God’s standards for any who tried to attain a right standing with God through works — with the end purpose of demonstrating that it couldn’t be done.
The fasting the Lord refers to here was fasting under the Old Testament law. During the Transition Period, fasting was voluntary (1 Corinthians 7:5). Paul never refers to it in his epistles written after the end of the Transition Period.