Paganism, Christianity and Buddhism are issues in Poul Anderson's works, e.g.:
Veleda, a sibyl of the goddess;
Axor, a Wodenite convert to Jerusalem Catholicism;
Adzel, a Wodenite convert to Mahayana Buddhism -
- so it is appropriate to say what we think about these traditions.
Paganism/Hinduism
While ritually celebrating seasons, solstices and life-stages, we can refer to gods and recognize them as personifications, not as persons. A skeptical philosopher can share a ritual with a hard polytheist. Neither attempts to interrogate or excommunicate the other. That is a whole 'nother tradition.
Christianity And Other Prophetic Monotheisms
Christianity is part of the conceptual transition between ancient and modern civilizations. In fact, monotheisms in general are transitional between polytheism and atheism although these different world-views coexist and interact. They do not mechanically succeed each other according to any conceptual scheme.
Buddhism
The Buddha identified and addressed an inner cause of suffering, tanha, variously translated as "grasping" etc. Each of us can practice meditation between now and death. Spiritually, that suffices. It is not necessary to believe either that rebirth occurs or that the purpose of meditation is to end rebirth. If that were true, then the Buddhahood of all living beings would mean the end of all life on Earth. I want to see the opposite: abundant life, consciousness, understanding and creativity. This accords with Poul Anderson's preferred future for humanity: freedom and diversity.