And it shall come to pass, that whosoever shall call on the name of the LORD shall be delivered. Joel 2:32
The prophet Joel describes how God is going to use the enemies of Israel to punish His chosen people for many centuries because of their rejection and unbelief in God. In spite of their idolatry, the Lord graciously asks them to repent of their sin and turn their hearts back to the Lord. God in His grace gives Israel and the nations, the opportunity to be saved from His righteous wrath, by calling on the name of the Lord. This is the promise of God to all who trust in Christ.
As believers we live by His grace. Although everyone who calls on the name of the Lord Jesus are saved by grace through faith in Him, this particular prophecy specifically relates to Israel and the coming great tribulation period when all who call on the name of Jesus will be saved from God's terrible wrath. A multitude of people will call on the name of the Lord but will be slain for their faith in Christ during this tribulation period. Only those who are saved to the very end of the tribulation will become citizens in the millennial kingdom.