4 For since, as I before said, there are various proclamations, listen, as in a figure, to the prophet blowing the trumpet; and further, having turned to the truth, be ready for the announcement of the trumpet, for he says, 'Blow the trumpet in Sion: sanctify a fast.' This is a warning trumpet, and commands with great earnestness, that when we fast, we should hallow the fast. For not all those who call upon God, hallow God, since there are some who defile Him; yet not Him— that is impossible— but their own mind concerning Him; for He is holy, and has pleasure in the saints. And therefore the blessed Paul accuses those who dishonour God; 'Transgressors of the law dishonour God.'
So then, to make a separation from those who pollute the fast, he says here, 'sanctify a fast.' For many, crowding to the fast, pollute themselves in the thoughts of their hearts, sometimes by doing evil against their brethren, sometimes by daring to defraud. And, to mention nothing else, there are many who exalt themselves above their neighbours, thereby causing great mischief. For the boast of fasting did no good to the Pharisee, although he fasted twice in the week, only because he exalted himself against the publican.
In the same manner the Word blamed the children of Israel on account of such a fast as this, exhorting them by Isaiah the Prophet, and saying, 'This is not the fast and the day that I have chosen, that a man should humble his soul; not even if you should bow down your neck like a hook, and should spread sackcloth and ashes under you; neither thus shall you call the fast acceptable.' That we may be able to show what kind of persons we should be when we fast, and of what character the fast should be, listen again to God commanding Moses, and saying, as it is written in Leviticus, 'And the Lord spoke unto Moses, saying, In the tenth day of this seventh month, there shall be a day of atonement; a convocation, and a holy day shall it be to you; and you shall humble your souls, and offer whole burnt-offerings unto the Lord.'
And afterwards, that the law might be defined on this point, He proceeds to say; 'Every soul that shall not humble itself, shall be cut off from the people. '
Source: Letters (New Advent)