There are few men more worthy of the title Great than Leo. He was pope in AD451 at the Council of Chalcedon, which was the council that put the death nail in the Arian controversy. The distance of time and history make it hard for us to believe the immense controversy that raged at this time.
The Arians believed that Jesus was not God, that he was created. The council was called to settle the issue. Leo wrote his famous "tome" which is basically a letter stating the church's position. This tome is probably the most important ever written in church history. It is a masterpiece of theology and logic and it is still the basic teaching of the church on Jesus.
"Roma locuta est. Causa finita est" (Rome has spoken. The cause is finished.) These are the famous words used by Leo to settle the argument and are the first time that the See of Rome took on itself the authority to settle a matter just on the wood of a pope.
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