God sent His Word made Flesh to us for our salvation. What Jesus Christ communicates, which is what the Father wants Him to communicate to us, is not a collection of niceties, wise sayings, non-binding agreements, or arbitrary recommendations. What He says and does has consequences for eternal life. “If you love me, you will keep my commandments” (John 14:15). Our Lord is quite clear.
God wills that this message of salvation be communicated whole and intact, with integrity and truth. To insure this happens, He establishes His Church and chooses apostles who are an essential component to what St. Paul will later refer to as the Body of Christ (cf. Ephesians 1:15-23; 5:23-30; Colossians 1:24). The twelve Apostles are an integral group. For this reason, when Judas falls away and betrays our Lord, he must be replaced.
We read in the Acts of the Apostles that “the brothers… a group of about one hundred and twenty persons” (Acts 1:15)—in other words, an official convocation of the Church—gathered to complete the twelve (Acts 1:16-17). Most revealingly St. Peter leads the group and conducts the proceedings (Acts 1:15). Two candidates are proposed (Acts 1:23). Following prayer (Acts 1:24-25), Matthias is selected by the casting of lots (Acts 1:26), a process mentioned numerous times throughout the Old and New Testaments. With this selection complete, Matthias “was counted with the eleven apostles” (Acts 1:26). Succession is important for the work and mission of the Church, the Body of Christ, and the bishops represent that succession. Here we encounter a continuity in the Church whereby “the bishops have by divine institution taken the place of the apostles as pastors of the Church” (Lumen Gentium, Second Vatican Council, 20 § 2).