In the first two verses Joel speaks out again that the day of the Lord is near because of the massive locust plague. It says that there were so many locusts swarming that they blackened the light. Joel compares this cloud of blackness to an approaching army that would bring great destruction along with it. Joel writes out a picture of what it appears like to him in verses 5-10. But when it comes to verse 11 we see just how strong our God is. He utters His voices and they follow and carry out His word.
Even though all this has happened and it is late it isn't too late for the people to repent and turn to Him. It was a common thing back then for people to tear their clothes when they were mourning or upset. But in verse 13 God doesn't want their garments, He wants their broken hearts. He is a forgiving, gracious, and a compassionate God. In verse 15 Joel says to blow the trumpet and make preparations for a fast. The priest were supposed to be in charge of getting the congregation together and heading up a group wide fast but back in chapter 1 we found out that the priest had abandoned their duties because of everything going on. In verse 17 all the nations around Judah were mocking them and God because they couldn't see how God could be a God who couldn't take care of His people during this rough time. They like so many didn't understand the ways of our God and just how merciful He was actually being toward them.
In these several verses the land of Judah are about to see just how gracious their God is. He is jealous for His land and has pity for them. He promises them that He will give them food, clear out the locusts, bring refreshing rain, and restore to them the protection over their land. Among many other things God restores to them what they have lost and reminds them that He is, was, and will always be there. His people will never be put to shame.
God would pour His Spirit on ALL the people of Judah to sustain them. Nothing in this contexts indicates that these people would proclaim anything or evangelize to the other nations but instead would have a close relationship with God. In the last verses we see that those who are faithful and call to God will be saved. There will be many signs around us to proclaim that the day of the Lord has come. It isn't a physical membership that brings us deliverance but only when we answer His call.