I don't think Wicca was ever against Christianity. Just a different belief.
Wicca is, very loosely, based on some of the old traditions of witchcraft. It was formulated as a religion by Gerald Gardner in an attempt to have some form of witchcraft made legal in Britain. He, together with others, succeeded when the practise of witchcraft became legal in 1951.
Wicca is a very new religion; but it is a religion; not strictly witchcraft.