Magick is an old English spelling of Magic which was revived by Aleister Crowley. He defined it as “the science and art of causing change in conformity with Will.” The crucial word in this sentence is Will. This does not mean any passing fancy, but refers to the great spiritual forces which are driving ones soul. Magick is therefore really about finding your Soul’s purpose – and then giving effect to it.
The concept of True Will is something Crowley took – like most things – from the Golden Dawn. True Will is what occurs when your ordinary everyday conscious Will is perfectly united with your Higher Will – which is your aspiration to that which is highest and most spiritual.
Wicca is a term most associated with the movement first brought to public consciousness by Gerald Gardner in the middle of the last century. It is primarily concerned with worship of the Goddess and God, and the observance of the traditional pagan festivals (Sabbats) and full-moon ceremonies (Esbats). There is now evidence to suggest that what we now know as the modern Wicca movement was founded in the 1920s by former members of the Golden Dawn who believed that they had been Witches in previous incarnations. Gardner did not found Wicca, but he was the first person to actively publicise it. See, for example, Gerald Gardner and the Cauldron of Inspiration by Philip Heselton (which coincidentally I once reviewed in the Journal of the Western Mystery Tradition).
Witchcraft is a general term for the historic Witch tradition. Many Wiccans would say that Wicca is Witchcraft, or at least a part or an example of Witchcraft: I do not particularly want to get into an argument upon the matter.
Can a Christian ever practice Magick – and remain a Christian? Certain elements of Thelema and Wicca have a religious character, so in these instances, probably not. However, one should also remember that for 1900 years prior to the 20th century, magick was being preserved and studied by Christian scholars. Not, of course, those who slavishly followed the dictats of the Church, but freethinkers who believed that the Kabbalah was the perfect synthesis between magic, mysticism and religion – even though at times they were persecuted by the mainstream Church for daring to say so.
So for a modern day Christian who is thinking of magick I would say if you are such a Free-Thinker then yes it is possible – you would then find Christian overtones in Martinism, the Elus Cohens, Waite’s Fellowship of the Rosy Cross, Dion Fortune’s Society of the Inner Light, and even in the Golden Dawn.