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From the Historical Introductions, "The 'Book of Concord, or Concordia, is the title of the Lutheran corpus doctrinae, i.e., of the symbols recognized and published under that name by the Lutheran Church. The word symbol, sumbolon, is derived from the verb sumballein, to compare two things for the purpose of perceiving their relation and association. Sumbolon thus developed the meaning of tessara, or sign, token, badge, banner, watchword, parole, countersign, confession, creed. A Christian symbol, therefore, is a mark by which Christians are known. And since Christianity is essentially the belief in the truths of the Gospel, its symbol is of necessity a confession of Christian doctrine. The Church, accordingly, has from the beginning defined and regarded its symbols as a rule of faith or a rule of truth. Says "Symbolum est regula fidei brevis et brevis numero verborum, grandis pondere sententiarum. A symbol is a rule of faith, both brief and brief, as to the number of words, grand, as to the weight of its thoughts.'"