Tales Of The Flying Mountains,
Prologue.
The Advisory Council of the spaceship Astra clarifies the need for selection and interpretation of historical facts. The examples given are reasonable. They could obviously be discussed further but here they are offered only as examples. "Mohammedanism" should be "Islam" but that is the kind of historical detail that is always with us.
"'...man being what he is, moral judgments are inevitable. Was it right, was it desirable that Christianity take over Europe...?'" (p. 16)
Do we need to discuss whether this was right or desirable? It happened and we certainly need to understand why. One question in a future history takes us back into past history and, within the works of the Andersons, to the times when merfolk fled from an exorcism or when the last King of Ys converted to Christianity.
I have a Religion and Philosophy blog for such questions but fewer people read it.
Possible Reasons Why Christianity "Took Over"
monotheism, with an omnipresent deity
a universal morality
freedom from divisive dietary laws
a single blood sacrifice of a perfect victim, efficacious for all time
a divine incarnation
a death and resurrection
a triune deity
fulfilled prophecies (the historical element)
a Mother of God able to replace the Mother Goddess
angels and saints instead of gods and heroes
the Adversary as a rebel angel instead of an anti-God
canonization of saints replacing deification of emperors
the bishop of Rome becoming Pontifex Maximus (an already existing post)
scriptural and Classical scholarship
incorporation of Greek philosophical schools
monasticism as a retreat from a hostile secular world
the flexibility to shed some of these elements at the Reformation
Yes, Christianity is comprehensive, robust and still with us.
Is this a diversion? No. Poul Anderson's works incorporate every issue.