Sect can be used as an insult thus:
DEROGATORY
a group that has separated from an established Church; a nonconformist Church:
Or as a noun thus:
NOUN
a group of people with somewhat different religious beliefs (typically regarded as heretical) from those of a larger group to which they belong.
From a Catholic perspective one can call all non-Catholic churches sects, but that is rarely done. Catholics usually refer to Protestant Christians as "separated brethren" and to Orthodox Christians as "brethren" - assuming one is discussing males or using the masculine pronouns to cover both males and females.