Church of Satan
Why would a member of the Church of Satan and magic get into a number of years in a relationship, hiding the fact? Can it work with one believing in God and the other doesn't?
Truthfulness is not a trait of Satan (John 8:44) and, quite frankly, one would not expect that trait from a member of the Church of Satan.
I don’t know what kind of relationship you have in mind with your second question. If it is marriage, I have difficulty imagining any kind of good relationship, like marriage, between a Christian and a member of the Church of Satan. Marriage is an intimate bond of husband and wife. The most important bond that can exist between them is that of the same faith: the Christian faith. When one of the spouses is actively supporting and following Satan, that person is going the opposite direction in life (and headed toward a different place in eternity) than his or her spouse.
Christians will want to warn Church of Satan members of the eternal fate of Satan and his followers (Revelation 20:10-15), share the saving gospel of Jesus Christ with them (Acts 4:12) and pray for them.