Answer:
The period between 90 AD and 400 AD saw the pinnacle of the Roman Empire, and also the seeds of it's downfall in the West, and its transformation in the East. And in this process Christianity would play a pivotal role leading to the establishent of it as the official religion if the Empire, and later to become the most powerful institution in Western Europe.
Explanation:
The Roman Empire saw the pinnacle of its power between the Flavian and Antonine dynasties, a period that correspond between the 69-192. However, condtions in the empire began to deteriorate and with the death of Commodus the last Antonine emperor, the Third Century Crisis began. Harsh times enabled a religious sentiment to flourish all over the Empire. Religion tend to grow in this epoch as a response to what it seems the break of the status quo with wars, invasions and social unrest. However, Christians were only one of many sects and cults, with others such as the Isis cult and the Mitriacs also flourishing, but diferred in the fact that they were monotheistic and did not comulgate with the cult of the Emperor. This would lead to persecutions and martyrdom, specifically under Domitian by 302 with the Great Persecution.
But the conditions of the Empire continue to deteriorate, and also the evolution of the Imperial government to a more centralized view of the world began to appeal of a monotheistic religion. Christianity was becoming the dominant religion, and rapidly becoming a more institutionalized one. But it would be the reign of Constantine that would initiate its establishment as an institutionalized religion with the Edict of Milan in 313 that ended legal persecution, and then the Council of Nicea in 325 that created a common framework for the Christian cult and purged alternative views such as gnosticism.
Paganism would have a last chance with Julianus the Apostate, but his premature dead in 363 lead to the consolidation of the Church with Theodosius the Great that by 393 had effectively banned pagan celebrations, and effectively making Christianity the only religion of the Empire. By 400, the old Roman panreligious pantheon was wiped out, and a new cesaropapist system, that would survive in the Eastern Roman Empire until 1453, was firmly established.