What are the core beliefs of orthodox Christians

I believe in one God,

the Father almighty,

maker of heaven and earth,

of all things visible and invisible.

I believe in one Lord Jesus Christ,

the Only Begotten Son of God,

born of the Father before all ages.

God from God, Light from Light,

true God from true God,

begotten, not made, consubstantial with the Father;

through him all things were made.

For us men and for our salvation

he came down from heaven,

and by the Holy Spirit was incarnate of the Virgin Mary,

and became man.

For our sake he was crucified under Pontius Pilate,

he suffered death and was buried,

and rose again on the third day

in accordance with the Scriptures.

He ascended into heaven

and is seated at the right hand of the Father.

He will come again in glory

to judge the living and the dead

and his kingdom will have no end.

I believe in the Holy Spirit, the Lord, the giver of life,

who proceeds from the Father [and the Son],

who with the Father and the Son is adored and glorified,

who has spoken through the prophets.

I believe in one, holy, catholic and apostolic Church.

I confess one Baptism for the forgiveness of sins

and I look forward to the resurrection of the dead

and the life of the world to come. Amen.

Basically the same shit as Catholics (mass, apostolic succession, sacraments, salvation can be lost by mortal sin and regained through confession) except they have beards and disagree on Purgatory, Mary, and the order of the Trinity and using Greek instead of Latin. Pretty much al Christianity in the entire world was Catholic/Orthodox prior to the Protestant Reformation of 1519.