The term far off includes both time and place. God’s promise extends throughout the generations until the end of the world. It also reaches people from every nation, tribe, race, and language, wherever they dwell on the face of this earth. Peter’s words are in complete harmony with those of Jesus: “Make disciples of all nations” (Matthew 28.19). And, second, God is sovereign in calling His own people to Himself. Salvation originates with Him and He grants it to all those whom He, by His sovereign grace, effectively will call. These words of Peter correspond to and have their counterpart in Joel’s prophecy: “And it will be that everyone who calls on the name of the Lord will be saved” (verse 21; Joel 2.32).
From: New Testament Commentary: Exposition of the Acts of the Apostles by Simon J. Kistemaker (Grand Rapids: Baker Book House, 1990), p. 107. Commenting on Acts 2.38-39.