I know that Constantine's political endorsement of the Church brought power over the dying paganism of the Roman empire, but the fact is that the Christian gospel did "out-think, out-live and out-die" the ancient pagan world, as T.R. Glover put it. A few thoughts: (1) The Unknown God was now revealed in our world, in human flesh; (2) The nature of God was astonishingly good and gracious in salvation; (3) The Christian gospel bridged transcendence and immanence, time and eternity; (4) The creation is being renewed by God from within, with the problem of evil being addressed in a daring way (God embracing the power of evil and suffering to defeat it from the human side); (5) there is hope, not a cycle or gradual decay to nothingness (the Resurrection and Return of Christ); and finally, (6) the dynamic presence of God the Holy Spirit in the lives of people, changing, healing, guiding. Sacrifice and ritual and offerings were ended by the cross of Christ.
If the ancient and early church had operated with the modern liberal theology so fashionable today, their message would have been greeted as essentially the same world-view as paganism itself in metaphysics and meaning.
Had the gospel been what some theologians want to make it, the church would have produced no martyrs, no persecution, no world-changing influence. There would be no church today; Christianity would have dwindled away into the blend of other similar panetheistic systems of thought.