Question

What issue was the most important cause of the split between the eastern and western halves of the Christian?

Answer 1

it was a dispute over the Nicene Creed. After it had been hammered out in 325 at the First Council of Nicaea, the Western Church held subsequent NON-ecumenical councils and added an extra word, "filioque", to the Creed, indicating that the Holy Spirit procedes from both the Father AND the Son. For the Greek speakers, this was nonsense and heresy.

The real reason was probably the dispute over jurisdiction. In the West, the Roman Empire had collapsed, leaving what was left of government in the hands of the bishop of Rome. To keep from being overwhelmed by responsibility, he standardized rules and practices, both secular and religious, throughout the West, and developed a hierarchical chain of command over which he presided, making all other bishops subordinate to himself. The results were satisfactory enough that he felt he should extend his organizing services to the patriarchs of the East. Unfortunately, as their half of the Empire still survived, they did not feel the need for a superior and rejected his offer. This pretty much destroyed any chance to preserve the unity of the Christian Church.

Political and cultural differences, as well as doctrinal issues was the cause of the split between the eastern and western halves of the Christian Church. One of the issues was the source of the Holy Spirit. They also disagreed on whether leavened or unleavened bread should be used during Holy Communion.

also the Western Church believed that their head should be the head of the entire Church, the Eastern Church disagreed.

Answer 2

there are many reasons so i just put a few.