Christianity gave no one customs and held little together. Europe has its own customs before Christianity that served as glue just as well, and the morality brought forth by christianity was so tenuous that you could make a good argument that it is even weaker than we have now.
If you want to see what religion being used to hold a society together looks like, look at the Middle East. The Middle Eastern religious elites are far more decadent than your tirade would predict, and society at large is not much better.
For all of your Wikipedia-summary characterization of Marx, he made almost the same argument as you have in substance, but far better, over 150 years ago when he wrote of alienation of labour and of the revolutionary aspect of capitalism.
Unlike you he actually did diagnose the central issue with "decadence" to the right source. Morality historically as well as societal norms were held up by the community, and the community was held up by its interdependence and shared experience. Marx foresaw that capitalism morphing these relations of interdependence into market relations, such as that the actual people that you rely on through shared labour are no longer apparent, and instead only money and products remain. This weakens the first social basis of community. Then, he saw capitalism as a mode of production, being entirely dependent on market relations as we saw earlier, as materially unshackled by the morality and structure of its host society. This is by contrast to feudalism that relied on these relations to perpetuate itself. From that he found capitalism to be revolutionary - it would continuously remake society after itself and in the process progressively destroy cultural norms of the society, thus being a revolutionary process. Sometimes it does so positively, sometimes negatively.
That is why Marx is said to be a materialist - instead of predicting the changes in society purely because of ideas and abstract concepts, he thought that the actual material limits on society would eventually come to dominate the ideas that people have and mold them moreso than the reverse. He wasn't a materialist in the sense that he thought that literally nothing except stuff mattered, much to the contrary, the stuff matters only because of its effect on people.
Now with this perspective in mine, if you re-read the statement, it should make even more sense.