One of the posters mentioned the fact there are many answers to this question. I agree.
Many would say prosperity is a cause for the decline in Christian spirituality in the western world. This trend is nothing new. The Jews faced it when they entered the Promised Land. God warned them: Deut. 6: 10-12:
10: And it shall be, when the LORD thy God shall have brought thee into the land which he sware unto thy fathers, to Abraham, to Isaac, and to Jacob, to give thee great and goodly cities, which thou buildedst not,
11: And houses full of all good things, which thou filledst not, and wells digged, which thou diggedst not, vineyards and olive trees, which thou plantedst not; when thou shalt have eaten and be full;
12: Then beware lest thou forget the LORD, which brought thee forth out of the land of Egypt, from the house of bondage.
Human nature has not changed any since then. You see when people have ALL the needs and wants supplied they feel no need for God. Have you never been tempted in this way? I have.
Another thing I have witnessed in my own denomination is when "Hellfire and Brimstone" preaching was being replaced by a self-centered gospel - a "God exists to make us happy" theology. In this theology people will discard Biblical truth which they deem to be unnecessary for moderns. In other words, moderns are guilty of violation of the first commandment: Thou shalt have no other gods before me." People have displaced God's word for one of their own making. When this happens, what else can you expect?
This is my two cents worth...
ginnyrose