Joel 2:15

If these [vessels] were approved by the Lord it was at a time when the priests had to offer victims and when the blood of sheep was the redemption of sins. They were figures typifying things still future and were "written for our admonitions upon whom the ends of the world are come." But now our Lord by his poverty has consecrated the poverty of his house. Let us, therefore, think of his cross and count riches to be but dirt. Why do we admire what Christ calls "the mammon of unrighteousness"? Why do we cherish and love what it is Peter's boast not to possess? Or if we insist on keeping to the letter and find the mention of gold and wealth so pleasing, let us keep to everything else as well as the gold. Let the bishops of Christ be bound to marry wives, who must be virgins. Let the best-intentioned priest be deprived of his office if he bears a scar and is disfigured. Let bodily leprosy be counted worse than spots on the soul. Let us be fruitful and multiply and replenish the earth, but let us slay no lamb and celebrate no mystic Passover, for where there is no temple, the law forbids these acts. Let us pitch tents in the seventh month and noise abroad a solemn fast with the sound of a horn.

(Verse 15 and following) Blow the trumpet in Zion: sanctify a fast, call a solemn assembly, gather the people: sanctify the congregation, assemble the elders, gather the children and those who still nurse at the breast. Let the bridegroom come out of his chamber, and the bride out of her closet: between the porch and the altar let the priests, the ministers of the Lord, weep and say: Spare, O Lord (Vulgate adds, spare), your people, and do not let your heritage become a reproach, that the nations should rule over them. Why do they say among the peoples, 'Where is their God?' LXX: Blow the trumpet in Zion, sanctify a fast, call a solemn assembly, gather the people, sanctify the congregation, assemble the elders, gather the children and nursing babes; let the bridegroom go out from his chamber, and the bride from her dressing room. Between the vestibule and the altar, let the priests, the ministers of the Lord, weep and say: 'Spare Your people, O Lord, and do not give Your heritage to reproach, that the nations should rule over them. Why should they say among the peoples, 'Where is their God?'" } Still he exhorts them to repentance, before the enemy army comes. Above, he said: Blow the trumpet in Zion, wail on my holy mountain, and so on, because the day of the Lord is coming, the day of darkness and gloom, the day of clouds and whirlwinds, announcing to you a numerous and powerful people that will come, which will overthrow your possessions and cities. Now because I am kind and merciful, patient, and abundant in mercy, I again command and say: Blow the trumpet in Zion, and preach repentance to the peoples; sanctify the fasting, preach healing, or gathering, of which we have already spoken: gather the people, so that the one who had sinned by being dispersed, having been gathered, may cease to sin. […]

Blow the trumpet in Zion,.... For the calling of the people together to religious duties, which was one use of the silver trumpets made for and blows by the priests, Num 10:2; sanctify a fast, call a solemn assembly; See Gill on Joe 1:14.

A promise that, upon their repentance, God would remove the judgment, would repair the breaches made upon them by it, and restore unto them plenty of all good things,

(3.) Their faith in God shall be confirmed and increased. When temporal mercies are made by the grace of God to be of spiritual advantage to us, and plenty for the body is so far from being an enemy (as with many it proves) that it becomes a friend to the prosperity of the soul, then they are mercies indeed to us.