Constantine's role was much bigger than that. For one thing, the Edict of Milan covered most or all religions -- not just Christianity. But Christianity flourished largely because Constantine himself adopted it as his own faith, and he ordered the Christian leadership to seat the Council of Nicaea to develop a unified doctrine regarding the Divinity of Christ.
I agree. My main point was that Western Civilization predates Christianity by a long, long time. And that even after Christianity came to Rome there were parts of Europe that were not Christian for almost 1,000 years. Some had quite developed civilizations, and a huge impact on European History.
I still think Constantine's greatest achievement is the founding on Constantinople (now Istanbul).