6.

 What early Christian writer clearly expresses this concept in seven short letters?

            This concept was expressed by an Apostolic Father, St. Ignatius the God-Bearer, Bishop-Martyr of Antioch (+107), who was the child whom the Lord took into His arms (Mt 18, Mk 9). He proclaimed the Gospel in Antioch and became the bishop of that city after the Apostle Peter. While on his way to his martyrdom in Rome (he was fed to the lions), he wrote seven epistles to Christian communities and to another Apostolic Father, St. Polycarp, a disciple of the Apostle and Evangelist John the Theologian and bishop of Smyrna (+156). These epistles contain a wealth of information on early Church dogma, organization, the Divine Liturgy, and the readiness for martyrdom.