Replacing Judas Iscariot
During the days of waiting in the upper room, Peter has stood up among the brethren to address a dark administrative necessity: the betrayal and death of Judas Iscariot. Peter correctly identified that Judas's defection was the fulfillment of Scripture, citing the Psalms to declare that his office must be given to another.
To maintain the foundational number of twelve apostles, representing the twelve tribes of Israel, Peter outlined the strict qualifications: the replacement must be a man who accompanied them during the entire earthly ministry of Jesus, from the baptism of John to the Ascension. Two men were put forward: Joseph called Barsabbas, and Matthias.
— Acts 1:21-22