Gratillonius meets St Martin of Tours on p. 62 of Poul and Karen Anderson, Gallicenae (London, 1988). Thus, a fictional character who is becoming a legendary figure meets a historical figure. (Gratillonius also meets a few other historical figures.) I visited a St Martin's Church in Birmingham recently.
Martinus travels around destroying temples - an abominable act. He is backed by the power of the Empire. Gratillonius reflects:
"...was it only, or also, that the Gods of the land were failing, that in some secret way folk knew they had no more reason to honour Them? Gratllonius remembered what Forsquilis had said in Ys..." (p. 64)
She had said that the old age was dying.
In Poul Anderson's The Boat Of A Million Years, the immortals convert to Christianity when expedient but then outlive all gods.