One of the fundamental differences between the Orthodox and Latin church was over a few small words in the Nicene Creed (in 325 A.D.) Here's how the Creed goes:
We believe in one God, the Father, almighty, maker of all things visible and invisible; And in one Lord Jesus Christ, the Son of God, begotten from the Father, only-begotten, that is, from the substance of the Father, God from God, light from light, true God from true God, begotten not made, of one substance with the Father, through Whom all things came into being, things in heaven and things on earth, Who because of our salvation came down and became incarnate, becoming man, suffered and rose again on the third day, ascended to the heavens, will come to judge the living and the dead; And in the Holy Spirit. But as for those who say, There was when He was not, and, Before being born He was not, and that He came into existence out of nothing, or who assert that the Son of God is of a different hypostasis or substance, or is subject to alteration or change — these the Catholic and apostolic Church anathematises.
[trans. from Early Christian Creeds by J.N.D. Kelly]
This was expanded in 381 at the first Council of Constantinople:
And in the Holy Spirit, the Lord, the Giver of Life, who proceeds from the Father; who with the Father and the Son together is worshipped and glorified; who spoke by the prophets. In one Holy, Catholic, and Apostolic Church. I acknowledge one Baptism for the remission of sins. I look for the resurrection of the dead, and the life of the world to come. Amen.
In A.D. 587, the local council of Toledo (Spain) added filioque to the Creed as an attempt to combat Arianism. (The Latin word filioque is translated in English as 'and the Son' and changes the Symbol of Faith from
And in the Holy Spirit, the Lord, the Giver of Life, who proceeds from the Father; who with the Father and the Son together is worshipped and glorified;
And 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 together is worshipped and glorified;
The reason for this was to combat the Arian heresy, which regarded the Son and the Spirit as subordinate to the father (the "first creation"). But this became a major difference between the Latin church and the Orthodox church. The Orthodox church held to the earlier formulation from the Nicene Creed.