Short answers, pulled from the story.
Theodosius I became Roman emperor in the winter of 379. He was chosen by the western emperor Gratian to take command of the eastern war effort after the previous emperor Valens was killed at the Battle of Adrianople.
The massacre of Thessalonica occurred in the spring of 390 when Roman troops slaughtered thousands of citizens in the hippodrome following a riot. The event exposed the volatility of Theodosius's temper and led to a confrontation with the bishop of Milan Ambrose who refused to give Theodosius communion until he performed public penance.
Theodosius I died in Milan in the year 394 after achieving his greatest military victory at the Battle of the Frigidus. His death marked the end of an era as the empire was immediately divided between his two sons Arcadius and Honorius and the Western Roman Empire would fall within the next century.
Theodosius I did not ban paganism nor did he systematically destroy temples but instead adopted a cautious policy that allowed pagan practices to continue while promoting Christianity as the state religion. He issued laws against animal sacrifice and divination but also allowed temples to remain open and even appointed pagans to high offices.
The Theodosian Renaissance was a period of renewed interest in classical art and culture during the reign of Theodosius I. He commissioned monumental works including the Forum of Theodosius the Column of Theodosius and the Obelisk of Theodosius which was brought from Karnak to Constantinople.