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The word literally means "one who is sent". The Lord himself is called an apostle as one sent by his Father (Hebrews 3:1 and cf John 20:21). However, the word "apostle" is generally used of the body of disciples, called "the twelve", to whom our Lord entrusted the organization of his church and the preaching of his gospel (Matthew 10:1-5; Mark 3:14-16; 6:7; Luke 6:13,14; 9:1; Acts 1:13). Our Lord gave them the "keys of the kingdom," and by the gift of his Spirit enabled them to build the foundation of his church (John 14:16,17,26; 15:26,27; 16:7-15). Judas Iscariot, one of "the twelve," eliminated himself from the number of the twelve, and Matthias was substituted in his place (Acts 1:21). Saul of Tarsus was afterwards added to their number (Acts 9:3-20; 20:4; 26:15-18; 1 Timothy 1:12; 2:7; 2 Timothy 1:11).
Luke has given some account of Peter, John, and the two Jameses (Acts 12:2,17; 15:13; 21:18) but beyond this we know nothing from authentic history of the rest of the original twelve. After the martyrdom of James the Apostle (Acts 12:2) James the Less usually resided at Jerusalem, while Paul, "the apostle of the uncircumcision," usually travelled as a missionary among the Gentiles (Galatians 2:8).
The qualifications of an Apostle are listed as follows:
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(Note: The answer above was adapted from an article in Easton's Revised Bible Dictionary)