Filioque

The Niceno-Constantinopolitan or Constantinopolitan Creed (which was written and adopted at the Council of Nicea in 325 AD, and then modified by the Council at Chalcedon in 451 AD) was later modified during the 6th century AD with the addition of the filioque.

This phrase states that they Holy Spirit proceeded from the Father and the Son. The Eastern Orthodox churches have historically rejected the filioque, citing John 15:26 as proof that the Holy Spirit proceeded only from the Father.

Friction over the filioque was a major cause of the split between the Roman Catholic Church and Eastern Orthodoxy in 1054 AD.