According to a Barna Survey, Denver is 49% "Post-Christian" which it defines has answering yes to nine of the following fifteen questions:
1. do not believe in God
2. identify as atheist or agnostic
3. disagree that faith is important in their lives
4. have not prayed to God (in the last year)
5. have never made a commitment to Jesus
6. disagree the Bible is accurate
7. have not donated money to a church (in the last year)
8. have not attended a Christian church (in the last year)
9. agree that Jesus committed sins
10. do not feel a responsibility to “share their faith”
11. have not read the Bible (in the last week)
12. have not volunteered at church (in the last week)
13. have not attended Sunday school (in the last week)
14. have not attended religious small group (in the last week)
15. do not participate in a house church (in the last year)
Only two other cities outside the Northeast and Pacific Coast had similarly high ratings: Tuscon, Arizona (50%) and Cedar Rapids, Iowa (49%). San Francisco at 53% was the most post-Christian city in the West. Albany, New York at 63% was the most post-Christian in the United States of the ninety-six cities surveyed.
If you look at the factors, one doesn't have to be all that secular to fit this definition.
At least five of the points, like "made a commitment to Jesus", a belief that "the Bible is accurate", "agree that Jesus committed sins" and participating in "a house church", and feeling "a responsibility to "share their faith" are distinctively Evangelical Christian doctrines or imperatives that many liturgical Christians might give the "post-Christian" answer to if asked in a survey. And, many adults would not have called a religious education function that the attended "Sunday school," even if they had attended it.
Thus, part of the differentiation regionally is denominational bias in the survey (which explains high number in the heavily Roman Catholic Northeast), while part of the differentiation is an actual regional difference in religiousity. Thus, while the highest percentages are in the Northeast, the West is probably actually more secular.
One could still be classed as "post-Christian" by the survey even if you have donated money to a church in the last year, have attended a Christian church in the last year, believe that faith is important in your life, have prayed to God in the last year, believe in God and do not identify as atheist or agnostic. And, not doing something "extra" beyond an ordinary church service every week in a liturgical church is hardly a "post-Christian" position.
Only about a quarter of people deemed "post-Christian" are "highly post-Christian" (twelve of more criteria) (9% overall v. 37% overall who are "post-Christian") - something that is not really possible if you are an even a Christmas and Easter liturgical Christian, although you could be a "none" who sometimes prays, rather than an atheist or agnostic, and fit that definition. The website doesn't break down this more accurate measure of "post-Christian" status by city.
Coming from Barna, the most interesting thing about the term is that it implies that Christianity is a past that will fade away in favor of a new non-Christian culture in an almost inevitable way.