Claim: Although the Orthodox Church may be the original Church, yet it has changed things since the time of the early Church, especially after Emperor Constantine.
Why this claim is false:
The Council of Nicea simply gave its stamp on what was already prayed and practiced in the early Church from the time of the Apostles' to the Council itself. The Nicene Creed is simply a composition of Scripture and the earlier Christian Creeds (cf. the creeds listed by St. Irenaeus of Lyons from 180 AD of what Christians believe, handed on from the Apostles, for example). The Emperor Constantine actually did not like the decisions of the Council of Nicea at first (he was inclined toward Arianism), but accepted them as the will of God through the gathering of the Bishops.
The fact is that all of the earliest church buildings are found in connection with Orthodoxy, the oldest "intact" church that we know of being the "old church" right below the Orthodox Church of St. George in Jordan (230 AD--see below), the Dura Church in Syria (235AD), the cave-sanctuaries of the Holy Land, Asia Minor, and the catacombs in Rome, all with Baptistries, a Holy Altar Table for the celebration of the Eucharistic Liturgy, and Icons.
For more on these matters, please see the links below:
The Oldest known text of hymn to St. Mary the Theotokos from 250AD (this hymn is still sung in the Orthodox Church to this day) Here is another ARTICLE on the same subject.
Icon of Christ the Sun of Righteousness and True Vine from 280 AD from Tomb under St. Peter's
Icon of Christ and Peter walking on water from 235 AD from Syrian Church
Icon of Christ Healing Paralytic from 235 AD From Church in Syria, in town of Dura
Icon of Christ as Good Shepherd from 230-240AD from Catacombs
For more on this topic please see our education website addressing further concerns for inquirers: https://www.orthodoxchristianed.org/answers.html