The English word "Church" comes from the German Kirche, which in turn comes ultimately from the Greek Kyriake, which means that which belongs to the Lord. However, in Scripture, the term is ekklesia, which in general meant a community called together. In the specific usage as God's Church, the Church is God's community called together in Christ.
The Church existed from long ago, throughout the Old Testament period. It included the Holy Prophets. Yet, by the grace of Christ it became His Body at Pentecost.
God's Church spread rather rapidly in the first few centuries. From the time of the Apostles until about the fourth century it subsisted in civil conditions that were generally hostile to it. However, with the legalization of Christianity in the early 4th century, and then it gaining favored status in the late 4th century by becoming the official religion of the Roman Empire, Christians could now worship more openly. The Church existed throughout the world as a single whole, with temporary schisms here and there, that often would be healed either as individuals or as groups reuniting with the whole Church. Originally the Church began at Jerusalem, and that was its geographical center in the early period. As it spread out, other centers arose. Already even in the New Testament period Antioch and Rome were added as centers.
The Church, therefore, throughout most of the first millennium had five centers: Old Rome, New Rome, Alexandria, Antioch, and Jerusalem.
Unlike other faiths, which claim that the church on earth is the "church militant" and that that which is "in heaven" is the "church triumpant, Orthodox Christianity holds to the original understanding of these terms. The Church is one Church. Thus, when we speak of "the Church militant" we speak of the entire Church in heaven and earth before the Last Judgment. That is because all strive for that end when all things shall be made subject to Him. In the General Resurrection, the entire Church becomes the Church triumphant. This we see in the book of Hebrews where those who came before "shall not be made perfect apart from us" (Hebrews 11.40).
*Note, the audio readings above are provided by Ancient Faith Radio and provides readings based on the Revised Orthodox Julian Calendar, because a majority of Orthodox jurisdictions are on the Revised Julian Calendar, as are the large majority of Orthodox in America. While most of the year the daily readings are the same for the Julian and Revised Julian Calendar, there is a portion of the calendar where they differ. For this reason, we here provide the readings for days when they differ.
The above audio readings are provided by AFR and are based on the Orthodox Revised Julian Calendar because a majority of Orthodox jurisdictions (and the largest three in the US) follow the RJC. There are no audio readings based on the old Julian reckoning of the Menaion.
The Orthodox Calendars (both Julian and Revised Julian) are both the same with regard to the Octoechos, the Lenten Triodion, the Pentecostorian (Flowery Triodion). The only difference is that observation of the Menaion (currently 13 days apart) and "shifts" that occur with regard to the Menaion (such as Lucan jump, and when major feasts fall on a particular day).