What does Acts 2:21 mean?

In this first sermon of the Christian church, Peter sets the standard for sharing the gospel with Jews. Around 900,000 Jews from around the Roman Empire have come to Jerusalem to celebrate Pentecost. Although they all probably speak Greek or Aramaic, most were raised speaking a minor, regional dialect. A crowd of these travelers has come upon a group of Galileans who seem to know every one of those dialects. Peter uses Old Testament prophecy—their own Scriptures—to explain what is happening. As Joel promised, the Holy Spirit has come upon the servants of God, giving them the ability to prophecy, or reveal truth (Acts 2:17–18). Now, Peter starts to show how this prophecy relates to Jesus.

This verse quotes Joel 2:32. Joel 2:28–31 describes how those who repent will be given the Holy Spirit and special messages from God while those who continue to reject God will see blood, fire, smoke, and a darkened sky. There are two distinct groups, and whoever calls on the Lord will be saved.

Those who watched Jesus die on the cross witnessed this recently. One of the thieves who hanged with Him trusted Jesus with his eternal soul. With no other work or admission, Jesus assured him he would go to paradise (Luke 23:39–43). Now Peter's job is to show how "the Lord" is not a hoped-for Messiah who will arrive in the future, but Jesus of Nazareth who was murdered fifty days before and rose again. For that, Peter will transition from the words of Joel to the words of David (Acts 2:25–35).

"Name" is from the Greek root word onoma. It encompasses more than what someone is called. This idea includes all thoughts, feelings, and images that come to mind when thinking of the person. The "name of the Lord" includes Jesus' character, sovereignty, authority, power, and deity. To call on or pray in Jesus' name is to place our needs in the hands of the God of the universe.