In so much as history is mostly written by its winners, so too do conquerors generally regard the religions of the vanquished as being 'of the devil' and having a mighty power in magical terms. This has the effect of heightening the perceived powers of the conqueror's own belief - a perception which can then be reinforced by continued successful persecution and the expropriation of that other religion's doctrines of faith.And so, the god of the overwhelmed faith is portrayed as a 'Devil' in by the victors. This was the precise methods exercised by Christianity against Wicca when it rose to Inquisitional Supremacy; and with practitioners of the Wiccan Way keeping very much to themselves, most of what we know about 'witchcraft' comes from strategically contrived Christian propaganda.
The most visible aspects of old pagan lore were the regular festivals. However, no matter what steps the church took to overawe and suppress the festivals, it was the destiny of the festivals to persist and in doing so remained visible and popular. In order to cope with this, the bishops hit upon the clever plan of upsurging these festivals by superimposing their own self-styled calendar; to the extent that today we still celebrate many of the important days of ancient Wiccan & Pagan lore without necessarily being aware of it.
Similarly, the emblems of pre-Christian heritage are still very much apparent: the holly, ivy, mistletoe, robins, pine cones and tree decorations of christmas; Easter eggs, bunnies, baskets, flowers and hot cross buns ... all emanate from their pagan originals.
Source: 'Realm Of The Ring Lords' by Laurence Gardiner