What does Acts 1:17 mean?

Peter is recounting the story of Judas Iscariot to the group of 120 Jesus-followers who are in Jerusalem, waiting for the Holy Spirit (Acts 1:5, 15). Judas betrayed Jesus to the chief priests and elders (Matthew 26:47–56), but he betrayed his fellow disciples, as well. He was "numbered among" them, meaning he was included in all the rollcalls of the Twelve disciples (Mark 3:16–19; Luke 6:13–16). He joined in their ministry when Jesus sent the Twelve out to teach, heal, and expel demons (Mark 6:7–13). He was included in the Twelve who would receive thrones in Jesus' kingdom and the responsibility to judge Israel (Matthew 19:28). Ironically, Isaiah pointed out that Jesus was "numbered with the transgressors" (Isaiah 53:12)—a reference to His death, but foreshadowed by His association with Judas.

But the Gospels also hint at Judas' serious character flaw. Six days before the crucifixion, Mary of Bethany anointed Jesus' feet with sweet ointment and wiped them with her hair. It was the day that the families first anointed the lambs they'd chosen to sacrifice for Passover. Judas, however, completely missed the symbolism. All he could think of was the three hundred denarii the perfume could have been sold for. In his grumblings, he mentioned the money should have gone to the poor. In reality, he held the disciples' money, and he wanted it for himself (John 12:1–8). In retrospect, it is little wonder that, days later, he sold Jesus to the Jewish leaders for silver (Matthew 26:14–16).

Peter knows that Judas' betrayal was predetermined by God, even planned by God (John 17:12). But he also acknowledges the personal side of the issue: how the people sitting in that room are affected. God's plan is perfect, but rarely easy; it's important to allow ourselves permission to feel that hurt, and to acknowledge it.