The US outlier may be explained by other variable. For example, church attendance may be positively or negatively correlated with wealth, depending on the state

The US outlier may be explained by other variable. For example, church attendance may be positively or negatively correlated with wealth, depending on the state, which suggests a more complex relationship, at least in the US. Besides being a very rich country (at least globally), the US have other very particular features, like multi culturalism, etc... There was also an interesting article in the economist about the more competitive market for religion in the US compared to other countries - I unfortunately cannot find it back.