Here are the biggest differences:
- The Orthodox deny papal infallibility and supremacy
- They deny that the saints and angels see the Beatific Vision
- They permit contraception
- They permit divorce and remarriage (you can divorce twice before you have to stick with someone)
- They deny the Immaculate Conception
- They deny the Filioque
- They deny the doctrine of original sin
- They deny purgatory
The things on that list are very important and if you knowingly reject the Catholic teaching on those things you put yourself in a state of mortal sin. So don’t convert to Orthodoxy. But toward the bottom of that list you see things that are on the border between being “different doctrine” and “different emphasis.” For example, although the Orthodox
say that they deny original sin, they do believe that human nature is weak in fighting temptation and is headed toward death, and that this is a result of Adam and Eve’s fall. To me, that’s all the doctrine of original sin says. (Look it up in the Catechism.) The Orthodox also deny purgatory, but they do believe that some of the dead are somehow waiting to get to heaven, and our prayers can help them in that in-between state. I wonder if that isn’t similar enough to our belief to qualify as being a purgatorial state. But classic Orthodox apologists deny that it is. On the other side of the “different emphasis” border there are these things:
- There is the essence/energies debate.
This one is difficult to get our heads around in modern times because it’s fallen out of discussion in Western circles. But God’s energies are the substance of His actions. Grace is a divine energy; it flows from God into His creatures. The Orthodox say that God’s energies are really distinct from God’s substance, but they are still uncreated and eternal. Classically, Catholic apologists have argued that this idea constitutes heresy because if you have any real thing that exists eternally alongside God, you have multiple eternal beings, which is polytheistic even if the things are impersonal beings like energies. So Catholics have historically said that God’s energies are just extensions of His own being. For example, when God pours grace into your heart, He pours Himself into your heart. So there’s a difference about whether God’s energies are extensions of His own essence or not.
- There is a difference about whether the Old Testament should be translated from the original Hebrew or the Greek Septuagint.
Catholics value going to the original languages in our Bible translations. The Orthodox love the Greek Septuagint translation of the Old Testament, which is fine, but they often seem to like it so much that they prefer the translation over the originals, similar to KJV-onlyism but more ancient. I have seen Orthodox prayer books that explicitly reject Old Testament translations from the Hebrew and claim that Orthodox should only use the Greek Septuagint (or a translation of
that) for the Old Testament. I don’t think the Church would demand that they adopt the Hebrew Old Testament if the Orthodox wanted to come back into union with the Church; but it is a little weird.
There are a few other different points of emphasis like the Orthodox not liking Aristotelian philosophy and not accepting the idea that doctrine can develop over time
a la Cardinal Newman’s theory of doctrinal development. But faithful Catholics can disagree about those things too, so I don’t think they make that big of a difference. The major ones I think I’ve mentioned by now.