The word “Church” comes from the Greek Eκκλησία, that means “assembly” or “congregation”. The word itself does not imply any specific model or system, save a group of people united by a common interest and vision.
During his earthly ministry, our Lord Jesus Christ did not leave a perfectly organized apparatus that we could catalogue as an institution in the same way as we conceive a Church organization today; but he left the basis for the developing of one fully equipped organization and spoke about it:
“And on this rock I will build my church, and the gates of Hades will not overcome it.” (Mt 16:18)
“If they still refuse to listen, tell it to the church; and if they refuse to listen even to the church, treat them as you would a pagan or a tax collector.” (Mt 18:17)
The Christian community of believers was initially made up by the Apostles of the Lord and the little number of converts who accepted Lord Jesus as Messiah. (Mt 16:16); later on after Pentecost, it grew in number and offices during the entire second part of the first century. By the beginning of the second century, the Universal Christian Church had expanded from Spain to India, and was made up by a perfect communion between different city churches (Rev 1:7; 2-3) in an Episcopal system.
This very Christian Church, that slowly started to be called “Catholic” (Kαθολικός), from internal as much as external sources (1), started to break up into four main groups. Firstly the Church of Assyria became isolated from the Roman Empire and consequently since then, lived an independent existence from approximately 381 CE. The Churches in communion with the Church of Alexandria became independent after the Council of Chalcedon in 451 CE; and finally the remaining of Christians separated in two groups, one in communion with the Church of Rome, and the other with the Church of Constantinople, in 1054 CE.
WHERE IS THE CHURCH NOW AND WHO IS A MEMBER OF IT
Apart from the doctrinal convictions of each Christian, historically the Universal Church of Jesus Christ is divided into five main groups: Roman Catholics, Eastern Orthodox, Oriental Orthodox, Assyrians Catholics and all Trinitarian reformed churches that adhere and profess the Nicene-Constantinopolitan Creed.
Even though some theologians from every above mentioned denominations do not accept the idea that any Baptism or Sacrament or any form of Grace at all are present within other Christian bodies apart from their own; the vast majority of theologians from all denominations tend to mutually recognized all Trinitarian baptisms, and the working of salvific Grace in all those who are validly baptized and remain faithful to the truths of the Creed. (2)
The Universal Church of Christ is composed physically by all those baptized Trinitarian Creed professing Christians in the world, regardless of sex, nationality, race or status.(3)
Spiritually, the Church of Christ is formed by all those Christians who possess the Holy Spirit and are truly counted by God as his of children. Who really is a member of the Spiritual Church, either on Earth or in Heaven, is only something that only God alone knows. On Earth, we can only assume in good faith that all professing Trinitarian Christians are.
(1) St Ignatius of Antioch, Letter to the Smyrnaeans, Ch.8
(2) Catechism of the Catholic Church, Art. 1213, 1226, 1246, 1256, 1267, 1271
Australian Catholic Bishops Conference, https://www.catholic.org.au/acbc-media/downloads/all-downloads/bishops-1/handbook-1/1389-recognition-of-baptism-1/file
Catholic Archdiocese of Johannesburg, https://www.catholicjhb.org.za/valid-invalid-baptism-roman-catholic-guide-2017/
Catechism of the Anglican Church, http://anglicansonline.org/basics/catechism.html
Coptic Church, The Church, http://www.copticchurch.net/topics/thecopticchurch/Intro_to_the_Coptic_Church_fr_yacoub_malaty.pdf, p. 303-307
(3) St Ignatius of Antioch, Letter to the Smyrnaeans, Ch.6
Gal 3:27; 1Cor 1:2; 12:31