What does Acts 1:22 mean?
A few days after Jesus' ascension (Acts 1:9), 120 of His followers are gathered in an upper room in Jerusalem, praying for guidance (Acts 1:13–15). In the midst of this particular meeting, Peter stands and reveals what the Holy Spirit has directed them to do. Jesus chose twelve disciples, but one of them, Judas Iscariot, is dead. It is the responsibility of this group to choose his replacement from those present. He must have been with them during the span of Jesus' ministry (Acts 1:21). Here, Peter specifies the timeframe: from Jesus' baptism by John the Baptist (Matthew 3:13–17) until Jesus' ascension.Why this particular timeframe? In Jesus' last commission to the apostles, He tells them, "you will be my witnesses" (Acts 1:8). Judas' replacement must have witnessed Jesus' ministry. At Jesus' baptism, God spoke down, "This is my beloved Son, with whom I am well pleased" (Matthew 3:17). Jesus summarized His miracles to John the Baptist's disciples in Matthew 11:5: "The blind receive their sight and the lame walk, lepers are cleansed and the deaf hear, and the dead are raised up, and the poor have good news preached to them."
Most importantly, the twelve apostles have to be witnesses to Jesus' resurrection. Jesus visited His followers for forty days after the crucifixion, showing them "many proofs" that He was alive (Acts 1:3). Paul will later explain that without Jesus' resurrection, there is no point to Christianity (1 Corinthians 15:12–19). Without the resurrection of Christ, we're just asking for troubles on earth with no promise of anything better after death (1 Corinthians 15:30–32).
Coincidentally, the period from Jesus' baptism to His resurrection is the only span covered by all four Gospels—Mark and John start at Jesus' baptism. The twelve apostles are still witnessing to us today.