What do you mean by secular?

There are three options.

1. Not belonging to clergy.
2. belonging to the state.
3. belonging to this age / world.

I dunno if this has not been achieved before since you connect secular to civilization, thereby indicating that the first has something to do with the other. I do not think so.

Civilization as a term was used to indicate the operation of law to make something civil and away from criminal. It came to mean in later periods an antonym to barbarity. So, basically: with or without the binds of law. And here is another thing to consider: barbarity is basically not shaved: the state of beard wearing. Actually, when you go to a shop to have a shave or a haircut, the person helping you with that is called a barber!

Confused much? That's your vocabulary for you.

So, to expound on the sentence: "Christendom gave rise to one of the first truly secular civilizations" you are saying: Christendom gave rise to an age where the wearing of beards was not deemed criminal by law?

Well, there are some examples. In the Frysian Law it reads that wearing long hair and a beard was something of a personal choice: a freedom. So, thanks to Pepin, the Frank.

Looking at it from the top line: Why did Christian Civilization die? it presupposes it is dead. I disagree. You only have to go to Syria, Russia, or other places to see it is not.

More specifically, why did it die in the West? I guess, for the same reason other societies with their own professed world view died. Brute force, mass forced conversions, it is quite simple. Actually, in Western countries we see Islamization. A friend of mine told me the other day, his daughter became a muslima.

One of the key issues in Western Civilization is "Unam Santam"/ "Romanus Pontifex". This did not come about in a vacuum. The real reason behind this Papal Declaration was the need for financing the Papal States.

The bottle neck was how to get around the usury inhibition? Well, the Church and the Toscan merchants (bankers) came up with an ingenious solution: Provide a loan with interest where the principal never would be paid back and the future income of the church would be collateral.

This paved the way for exorbitant lifestyles of the Church Princes and sale of Indulgences. This then made Vatican Banking profitable and in turn was a huge impulse for worldly Princes and Kings to do the same, making taxes mandatory.

So, here, you see the death nail, a coupe the grace, to virtue and therefor the most basic of the Christian principles: personal accountability in the sight of God. In it's place came the structure, enforced by financing needs.

Although the reformation has responded harshly to this method, it was irreversible, as especially the protestants allowed this system to fester in their own countries. Heck, the Dutch were the first to reap the rewards, and later the Anglican English.

However, traditionally: it was not done to make a slave out of your fellow even IF he owed you, as the duty to preserve freedom for all individual members of the community was of a higher order, as was the duty to help your fellow to preserve it for him self.

And this is the fundamental threshold where wants, interests, and lust become destructive.