Why is New England so Irreligious?

First of all, I'm not trying to offend or stereotype anyone. When you look at the statistics, and just personal experience, people in the "north" are very nonreligious. Vermont, Machusetts, New Hampshire, Maine, Delaware, etc. have over 20% Irreligious among the population. While other states, such as North Dakota, Mississippi, Oklahoma, etc. have very low irreligious people. So my question is why? Is it totally due to the density of the wealthy, liberal, and highly educated people of those states? The reason this intrigues me is because most northern states were founded on religious properties. Pennsylvania (15%), Plymouth Rock in Machusetts, Rhode Island (19%), etc. Are there any concrete reasons for this? Please no rude or anti-religous answers, just opinions and data. Thanks.