Acts 1:22-26 The Apostle Judas was replaced

^{Jesus Christ ordaining the 12 Apostles}

²² Beginning from the baptism of John, unto that same day that he was taken up from us, must one be ordained to be a witness with us of his resurrection. ²³ And they appointed two, Joseph called Barsabas, who was surnamed Justus, and Matthias. ²⁴ And they prayed, and said, Thou, Lord, which knowest the hearts of all men, shew whether of these two thou hast chosen, ²⁵ That he may take part of this ministry and apostleship, from which Judas by transgression fell, that he might go to his own place. ²⁶ And they gave forth their lots; and the lot fell upon Matthias; and he was numbered with the eleven apostles. -Acts 1:22-26 Bible, King James Version

When Jesus established the Christian Church it had a foundation of twelve apostles. After the apostle Judas died he needed to be replaced so they chose two suitable replacement apostles and cast lots to decide which one to ordain in order to maintain the Church's foundation of twelve apostles.