God will pour out His Spirit upon the people to such an extent that seemingly everyone will be prophesying, dreaming, or seeing visions. This did not happen anciently to Judah, though it did occur in a more limited way on Pentecost after Jesus Christ died (Acts 2). This is speaking about the conversion of many people, and by their conversion through the receipt of God's Spirit, they become “His people.”

As a matter of speculation, there is a way of looking at the timing of the end-time fulfillment as a literal locust plague before the Great Tribulation. For instance, the several years it takes for grape vines and fruit and olive trees to mature and produce in abundance could indicate that the Tribulation is at least several years off when the locust invasion (mentioned in Joel 1:4; 2:25) strikes. Then come the “wonders in the heavens and in the earth: blood and fire and pillars of smoke. The sun shall be turned into darkness, and the moon into blood, before the coming of the great and awesome day of the LORD” (Joel 2:30-31). These signs (the sixth seal; Revelation 6:12-17; Luke 21:11) are the precursors to the great day of God's wrath.

The last verse, Joel 2:32, is most interesting, as it points out that those who call upon the Lord shall be delivered and saved. Here is how James Moffatt translates it: “But every worshipper of the Eternal shall be saved, for Sion hill shall hold those who escape, as the Eternal has declared, and the fugitives whom the Eternal calls shall be inside Jerusalem.” The Amplified Bible renders it: “And whoever shall call on the name of the Lord shall be delivered and saved, for in Mount Zion and in Jerusalem there shall be those who escape, as the Lord has said, and among the remnant [of survivors] shall be those whom the Lord calls.”

Who are those fugitives who escape and from what are they running? Who are those whom the Eternal calls? Revelation 12:15-17 prophesies of a remnant of commandment-keepers, presumably the church, fleeing from a flood of Satan's persecution. In fleeing, where did they go? In this speculative scenario, they go to Jerusalem before the army of locusts attacks and perhaps before the drought takes hold. They are in Jerusalem during that catastrophe where, apparently, there is food and water.

These church members could be the fugitives and those whom God calls. However, at this point their refuge is not what we call the Place of Safety, as the Tribulation has not yet begun. Those who are at the time in Judea, which includes Jerusalem, are later warned to run for the protection to be found in the mountains just as the Tribulation commences (Matthew 24:16-20).
Keeping in mind that Scripture was not written to the world at large, but to the few who have been called, it is not a great leap of faith to realize that these people delivered at Jerusalem are true Christians who flee from Satan's “flood.” The same group is warned to flee Judea to a safe place in the nearby mountains.