The main Christian doctrinal truth is the understanding of God as the Holy Trinity - the Father and the Son and the Holy Spirit. People who confess God in this way are called Trinitarians.
Indeed, Christians are only those who profess the Trinity of deity. There are three branches of Christianity: Orthodoxy, Catholicism and Protestantism. In all of these denominations, God is a Trinity: Father, Son, and Holy Spirit. However, there can be a difference in intra-trinity theology. For example, the Orthodox say that the Holy Spirit comes from God the Father, and Catholics add that the procession of the third hypostasis of the Holy Trinity comes from the Father and the Son. This is the so-called "filioque" insert, which at one time (even before the separation of the Churches in 1054) was added to the Nicene-Constantinople Creed.
In addition, one can mention the so-called pre-Chalcedonian churches, for example, the Coptic church, the Armenian church and several others, which did not accept the decree of the IV Chalcedonian Ecumenical Council. These Christians are neither Orthodox nor Catholics, nor are they Protestants. The dogma of the Trinity of deity is maintained in the pre-Chalcedonian Churches. However, there is some disagreement with orthodox Christianity regarding the natures in Jesus Christ. Thus, at the IV Ecumenical Council, the dogma was formalized that in Christ there are two natures - the divine and the human. The council was called over controversy over the human being in Christ. Opponents of the Council of Chalcedon argued that in Christ there is only one nature. Do-Chalcedonian churches still hold this opinion.
Now it is worth mentioning the sects, some of which consider themselves Christian. For example, Jehovah's Witnesses (people of Protestantism, a totalitarian sect of the Western type) do not adhere to a Trinitarian view of the essence of deity. That is why this organization is non-Christian. In the same categories one can speak about other sectarians and representatives of various currents of pseudo-Christianity.
Thus, it turns out that Christians in the full sense of the word are those who profess the Trinity of deity. He who is not a Trinitarian (does not profess the Trinity of God) cannot in the full sense be called a Christian.
