There are several things needed to create a Humanist culture that can effectively replace religious culture. The first of these is a common mythology, but not in the sense of a bunch of made up stories. Rather, these are culturally shared stories that may be a bit idealized or simplified. For Humanism, two obvious examples would be the story of human life evolving, and the second would be Darwin's development of the theory of evolution.
The second thing that needs to be created is a common morality that is practical and understandable. The three Humanist Manifestos are a good start, but are too abstract and vague in nature to be sufficient. Instead, the morality should consist of simple principles demonstrated in examples and backed up by justification. The existence of moral gray areas also should be addressed.
Third, a set of common, purposeful rituals need to be developed. For example, a community ritual (like going to church on Sundays) could help connect Humanists and allow Humanists to develop their understanding of Humanism. Acknowledging your own place in yourself, your relationships, society, the world, and universe at some regular, predetermined times (say after waking and before falling asleep) to help keep your life in context of your beliefs. There are countless other rituals that could be used.
Fourth, Humanism needs to clearly state why and how a life can be purposeful without a need to defer to the divine. This may sound simple, but this needs to carry enough weight that it can stand next to the divine arguments made by religions.
Finally, Humanism needs a clear and formal set of threats and promises to differentiate it from religions. These can be based off of a Humanist view of the future, combined with the shortcomings of religion. Threats and promises create incentive for people to convert, and also act to combat the argument that an atheistic belief system is inherently nihilistic.
So what would these things do? It would make Humanism more appealing and useful for its adherents and potential converts. It would also allow Humanism to engage in dialectical relationships with the more reasonable manifestations of religion. Finally, it creates a pseudo-religious religion alternative that isn't lacking (after leaving Christianity 6 years ago, I still miss many aspects of having a church).
Also, I don't think this list is complete or infallable, nor do I have any good ideas for implementation. As always, feedback is much appreciated, either directly or using the comment system.
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