It legalizes Christianity, which is a big deal because these folks have been persecuted all this time. They haven't been worshiping the Roman gods. Then in 325, Constantine goes even further, and he convenes the Council of Nicaea where he brings the bishops from all of Christendom together to start having a more unified belief system. They come up with the Nicaean Creed. A lot of the debate is whether Jesus as son of God should be considered equal to God or as God or a little less as being the son. Maybe if he's a son, maybe there was a time where there wasn't a son and there was just God. But the Nicaean Creed says no, they are one and the same.