The choice fell to Matthias.
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The paragraphs in Acts 1 immediately before these tell the story of the disciples witnessing Jesus' Ascension into Heaven, then returning to Jerusalem. The only activity described is constant prayer.
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Twelve is the number of the tribes of ancient Israel, each historically led by a patriarch, then by a judge. These Jewish followers of Jesus are still thinking in fairly strictly Jewish terms.
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They have yet to experience Pentecost, when people from all the ends of the earth are to hear these men preach as if in multiple languages.
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They're still thinking in terms of fulfillment of ancient promises, not in terms of entirely new promises and prospects.
