Calvin's Commentaries, Vol. 27: Joel, Amos, Obadiah, tr. by John King, [1847-50], at sacred-texts.com
Calvin’s Preface to Joel
I proceed now to explain The Prophet Joel. The time in which he prophesied is uncertain. Some of the Jews imagine that he exercised his office in the time of Joram, king of Israel, because a dreadful famine then prevailed through the whole land, as it appears evident from sacred history; and as the Prophet record a famine, they suppose that his ministry must be referred to that time. Some think, that he taught under Manasseh, but they bring no reason for this opinion; it is, therefore, a mere conjecture. Others think that he performed his office as a teacher not only under one king, but that he taught, at the same time with Isaiah, under several kings.
But as there is no certainty, it is better to leave the time in which he taught undecided; and, as we shall see, this is of no great importance. Not to know the time of Hosea would be to readers a great loss for there are many parts which could not be explained without a knowledge of history; but as to Joel there is, as I have said, less need of this; for the import of his doctrine is evident, though his time be obscure and uncertain. But we may conclude that he taught at Jerusalem, or at least in the kingdom of Judah. As Hosea was appointed a Prophet to the kingdom of Israel, so Joel had another appointment; for he was to labor especially among the Jews and not among the Ten Tribes: this deserves to be particularly noticed.
Now the sum of the Book is this: At the beginning, he reproves the stupidity of the people, who, when severely smitten by God, did not feel their evils, but on the contrary grew hardened under them: this is one thing. Then he threatens far more grievous evils; as the people became so insensible under all their punishments, that they were not humbled, the Prophet declares that there were evils at hand much worse than those they had hitherto experienced: this is the second thing. Thirdly, he exhorts the people to repentance, and shows that there was required no common evidence of repentance; for they had not lightly offended God, but by their perverseness provoked him to bring on them utter ruin: since, then, their obstinacy had been so great, he bids them to come as suppliants with tears, with sackcloth, with mourning, with ashes, that they might obtain mercy; for they were unworthy of being regarded by the Lord, except they thus submissively humbled themselves: this is the third subject. The fourth part of the Book is taken up with promises; for he prophesies of the Kingdom of Christ, and shows, that though now all things seemed full of despair, yet God had not forgotten the covenant he made with the fathers; and that therefore Christ would come to gather the scattered remnants, yea, and to restore to life his people, though they were now lost and dead.
This is the sum and substance. But we shall see, as we proceed, that the chapters have been absurdly and foolishly divided. He thus begins —