In 'The Crisis of Civilization', Hilaire Belloc, correctly states that culture is King. If your society believes that all cultures are equally esteemed and should be affirmed as relatively the same, no vital force and impetus will exist to propel civilization, on all of its various planes of existence, to 'broad sunlit uplands' of development. Decay and rot will set in. The Roman and Hapsburg polycultural empires make that very clear. Not even the veracity and energy of Christianity could have saved Rome. Contrary to Gibbon, its decline had begun long before the 4th century.
“But what was the internal force? How were men convinced ? Why did they join [Christianity]...Often it meant ruin...and sometimes torture and death....A man became Christian because he found that the Church affirmed things which he recognized to be true in experience and holy in character. It was loved, witnessed to and defended to the death by those who thus felt it to be, when in contact with it, divine...It was not society which proceeded from the doctrine, but the doctrine that came from the society.”
Culture creates civilization. If your culture preaches Evolutionary-survival of the fittest and raciology [Nazism], you will manufacture a death cult. If your society elevates scientist naturalism and evolution [Communism], you will produce the same immorality, violence, and human-hatred as the cult of Evolutionary-raciology. Once you denude Europe and North America of Christian doctrine confirming for example, that a Meccan celestial cult whose own artifacts and leaders preach and have preached Jihad, death, war and the visitation of destruction upon non-cult members, your own end is rather assured. It is a question of time.
Without the Catholic Church, the 'legacy' of Greece and Rome with most of the 'experts' ignoring their vast infirmities, calamities, slavery and human debasement; would never have survived. None of it. From the 4th century onwards the Church scooped up the breaking pieces of the Roman-Greek tradition and merged them with her own culture.
Belloc: “But the truth of the business....is almost the opposite of what has been said so long. It was not the spread of the Faith which undermined the high civilization of pagan antiquity; on the contrary, the Faith saved all that could be saved; and, but for the conversion of the Roman Empire, nothing of our culture would have remained.”
The above is obviously true. The decline of Rome into Germanic kingdoms was a blessing and long overdue. Inflation, taxation, bureaucratic corruption, abortion, slavery, 90% of the population in penury, a retardation post the 1rst century in technology, a declination in arts and architecture, not to mention the paucity of literature and science....the Roman empire by 250 AD had long outlived its usefulness. Thankfully there was an institution which could acquire the main artifacts, integrate them into a better society, and carry their legacies onward.