Complex sentence: The Church's teaching on marriage is that it is a Sacrament and that it is only validly contracted by the two individuals, so questions may arise as to whether that person is able to contract a valid marriage.
Split into simpler sentences: The Church's teaching on marriage is that it is a Sacrament and that it is only validly contracted by the two individuals. Questions may arise as to whether an individual person is (was) able to contract a valid marriage.