The Platonists and their Christian successors held the peculiar notion that the Earth was tainted and somehow nasty, while the heavens were perfect and divine. The fundamental idea that the Earth is a planet, that we are citizens of the Universe, was rejected and forgotten.
The Christian that is bound by his own horizon, the church that lives simply for itself, is bound to die a spiritual death and sink into stagnancy and corruption. We never can thank God enough for giving us not only a whole Gospel to believe, but a whole world to give it to.
No people are true Christians who do not think constantly of how they can lift their brother and sister, how they can assist their friends, how they can enlighten mankind, how they can make virtue the rule of conduct in the circle in which they live.