Today marks the day in 380 that western-style early Christianity got the government's approval as the favored flavor of state religion.
Recall that Constantine had re-labeled the Roman Empire as pro-Christian back around 313, but there were already several competing versions, and the Nicene one, which turned into the Catholic Church eventually, wasn't either the earliest or the popular favorite. The 325 Council of Nicaea didn't settle things down, and it wasn't until this day in 380 that Emperor Valentian and his sons issued an Edict that made it easier to be a fan of the Nicene version and harder to be devoted to others, like Arianism.
After the edict, Theodosius spent a great deal of energy trying to suppress all non-Nicene forms of Christianity, especially Arianism, and in establishing Nicene orthodoxy throughout his realm. In the following couple of years, the other sects lost the right to meet, ordain priests, or spread their beliefs. Theodosius forbade heretics to reside within Constantinople, and in 392 and 394 confiscated their places of worship.