Someone wrote this on the main page so I thought this could relate and answer your question as well since this is explained so well.
"The theory is Constance Langdon is a descendant or distant relative of Mimi DeLongpre, a Supreme and Anna Leigh Leighton's predecessor. This would make Constance and Tate carriers of the witch gene that was then passed on to Michael. At least that's the theory anyway.
My thoughts on this, however, is that Michael is not a witch at all. People are mostly basing their assumption on the fact that he is accepted by the sons of Salem and that he can perform their magic (but let's also remember the fact that the Demon that haunted the Asylum demonstrated at least three of the Seven Wonders so that magic isn't exclusive to witches). In fact, his magic is vastly superior compared to theirs. They believe this is simply because he is a "Level 4" warlock, the male equivalent to the Supreme and more powerful than the average warlock. They don't know that he's the Antichrist and they don't know who his "Father" is (nor do they care to know because they willfully ignore how dark Michael's magic is). BUT WE KNOW.
My only issue with us saying that he's a witch is that we're ignoring something about Michael that we know that the other characters who call him a witch don't know."