What did Constantine do that changed history?
Constantine divided the Roman Empire into Western and Eastern Empires. He legalized Christianity and oversaw the first Counsel at Niscia.
Constantine moved his capital from Rome to what is now Istanbul. This established Constantinople as a new center of civilization.
Rome and Western Roman Empire would be overrun by Germanic tribes by 450 AD. Western Europe would struggle to maintain a literate society and desend into what is often called the dark ages.
The East Roman Empire started by Constantine would last until 1452 when it was destroyed by the Muslim Empire of the Ottoman Turks.
The Eastern Roman Empire speaking Greek would preserve the knowledge of the Roman and Greek Civilizations and pass the information back to western Europe during the Crusades.
Also the Greek Orthodox Church that came out of the Eastern Roman Empire would convert and influence the development of the civilizations of Russia, Serbia, and most of Eastern Europe.
Constantine embraced Christianity legalizing the religion and making Christianity the official religion of the Roman Empire. This legalization helped Christianity to spread and become the dominate religion of both Western and Eastern Europe.
Without Constantine Western civilization might have died with Germanic and Hun invasions of the 400's AD. The knowledge of Greek and Roman civilizations for the most part lost. Christianity might not have become the dominate religion of Europe and the largest religion in the world.