Forgive me, I misspoke. The issue wasn’t Immaculate Conception, it was the Filioque, I got it all muddled in my head.
809 Pope Leo III forbids addition of Filioque to Creed and has original Creed in both Greek
and Latin inscribed on silver tablets displayed in Rome.
1014 First use of Filioque by Pope of Rome, at coronation of Holy Roman Emperor Henry II
The general problem is the development of doctrine. Things that weren’t heretical in 1400 are heretical now.
I also found a more complete list of what would be required in order for the Orthodox and Catholics to reunite (from the Orthodox perspective)
"In order for the Orthodox Church to accept sacramental communion with the Roman Catholic Church,that is, for Roman Catholics to be readmitted into the Orthodox Church, they have to repudiate and reject (not merely brush aside or “theologize around”) the following:
- Papal Universal Jurisdiction
- Papal Infallibility
- Papal Petrine exclusivism (i.e., that only the Pope is Peter’s successor)
- Development of Doctrine
- The Filioque
- Original Sin understood as guilt transmitted via “propagation”
- The Immaculate Conception of Mary
- Divine Simplicity
- Merit and Satisfaction soteriology
- Purgatory and Indulgences
- Created grace
Roman Catholics would have to accept and fully confess:
- The authority of Ecumenical Councils over the Pope
- The Essence/Energies distinction
Roman Catholics would have to restore Orthodox practices (already present for Eastern Catholics):
- Reconnect Confirmation/Chrismation back to Baptism rather than delaying it
- Give Holy Communion to all Church members, including infants
In other words, what the Orthodox expect of Roman Catholics is that they become Orthodox again"