Not the bare outward act
Not that we are to imagine the performing the bare outward act will receive any blessing from God. Is it such a fast that I have chosen, says the Lord: a day for a man to afflict his soul? Is it to bow down his head as a bulrush and to spread sackcloth and ashes under him? Are these outward acts, however strictly performed, all that is meant by a man's afflicting his soul? Will you call this a fast and an acceptable day to the Lord? No surely. If it be a mere external service, it is all but lost labor. Such a performance may possibly afflict the body. But as to the soul it profits nothing (Isaiah 58:5).
1771 text
3. Not that we are to imagine, the performing the bare outward act, will receive any blessing from God. Is it such a fast that I have chosen, saith the Lord: a day for a man to afflict his soul? Is it to bow down his head as a bulrush, and to spread sackcloth and ashes under him? Are these outward acts, however strictly performed, all that is meant by a man’s afflicting his soul? Wilt thou call this a fast, and an acceptable day to the Lord? No surely. If it be a mere external service, it is all but lost labour. Such a performance may possibly afflict the body. But as to the soul it profiteth nothing.