Which church" The Roman one or the EO one who holds to different doctrines and considers the themselves the OTC and the Roman rite off the rails.
They disagree in the following areas.
How can they both be true?
These differences are so important that there has been no reconciliation in nearly a thousand years after the split. The Eastern Orthodox differ with Roman Catholicism on these issues:
The Holy Spirit (the filioque)
In EO - The third person of the Trinity, proceeding from the Father alone as in the original Nicene Creed. The Father sends the Spirit at the intercession of the Son. The Son is therefore an agent only in the procession of the Spirit.
In RC - 'When the Holy Spirit proceeds from the Father and the Son, He is not separated from the Father, He is not separated from the Son'.
Mary - Assumption and Immaculate conception of
EO - The Assumption is accepted and it is agreed that Mary experienced physical death, but the Immaculate conception is rejected. Orthodox belief is that the guilt of original sin is not transmitted from one generation to the next, thus obviating the need for Mary to be sinless.
RC - Both are dogmas of the church. The church has not as yet decided whether Mary actually experienced Physical death. The dogma of the Immaculate Conception states that Mary, was at conception 'preserved immaculate from all stain of original sin' and should not be confused with the virgin birth.
Pope - Authority of
EO - As the Bishop of Rome, he has a primacy of honour when Orthodox, not of jurisdiction. At present, his primacy is not effective as the papacy needs to be reformed in accordance with Orthodoxy. His authority is thus no greater or lesser than any of his fellow Bishops in the church.
RC - The Pope is the 'Vicar of Christ' i.e. the visible head of the church on earth and spiritual successor of St. Peter. He has supreme authority (including that over church councils) within Christendom (The Power of the keys).
Pope - Infallibility of
EO - Papal Infallibility is rejected. The Holy Spirit acts to guide the church into truth through (for example) ecumenical councils. This Orthodoxy recognises the first seven ecumenical councils (325-787) as being infallible.
RC - The Pope is infallible when, through the Holy Spirit, he defines a doctrine on faith and morals that is to be held by the whole church. This is a dogma and is therefore a required belief within Catholicism.
Purgatory
EO - An intermediate state between earth and heaven is recognised, but cleansing and purification occur in this life, not the next.
RC - A place of cleansing and preparation for heaven. Also a place where the punishment due to unremitted venial sins may be expiated.
I'd say these were the "biggies", but other differences also exist. These are explained
And this article is another comparison.
Comparison between Orthodoxy, Protestantism & Roman Catholicism:
And yet the Catholic here claim no split! That’s a split for sure!