This is a question Christians ask each other across centuries of pain. The honest answer requires both charity and clarity.
What we share
The Nicene-Constantinopolitan Creed (381) is confessed by Catholics, Orthodox, and the vast majority of Protestants. We share: one God in three Persons, the Incarnation, the virgin birth, the atoning death, the bodily Resurrection, the second coming, and the canonical Scriptures (with some differences on the deuterocanonicals).
Where we differ
- Catholic vs. Orthodox: papal authority, the Filioque ("and the Son"), purgatory, the Immaculate Conception. Sacraments and apostolic succession are shared.
- Protestant vs. Orthodox/Catholic: sola Scriptura, the nature of the Eucharist, the role of Tradition, salvation language, sacraments, ecclesiology.
These are not trivial. The Orthodox Church holds that the fullness of the faith handed down by the apostles is preserved in Orthodoxy. But that conviction does not require denying that grace works among other Christians.
The Orthodox lens
St. Photios and the Fathers of the 8th Ecumenical Council named real disagreements. At the same time, the Orthodox liturgy prays for "the union of all" — not as polite ecumenism but as longing for what was torn. We can confess our differences honestly without claiming the Spirit has never worked outside our walls.
A pastoral word
If you came to Christ through a Protestant church, your love for Jesus is not invalidated by the question of where the apostolic deposit is most fully preserved. Orthodoxy invites you to "come and see" (John 1:46) — not to rebuke what got you here, but to offer you the fullness of what the apostles handed down.