Complex sentence: Certain groups have been labeled as cults and opposed by the Christian countercult movement for their unorthodox beliefs; and since the 1970s by the secular anti-cult movement, partly motivated in reaction to acts of violence committed by members of some groups.
Split into simpler sentences: Certain groups have been labelled as cults and have been opposed by the Christian countercult movement for their unorthodox beliefs. Since the 1970s this has also been true for the anti-cult movement, partly motivated in reaction to acts of violence committed by members of some groups.