Which theme is one of the major focuses in the book of Malachi?

The book of Malachi primarily addresses issues related to faithfulness, both in relationships and in the covenant community's relationship with God, but it does not exclusively focus on marriage and divorce as a primary theme. One of the prominent concerns in Malachi is the faithfulness of the people to God, juxtaposed against their unfaithfulness, which can manifest in various aspects of life, including relationships.

Marriage, particularly the sanctity of it and the implications of divorce as addressed in Malachi 2:10-16, is indeed discussed, but it serves as part of a larger context concerning the people's overall faithfulness. The prophet calls out the people for their general spiritual apathy and disobedience, illustrating how these problems manifest in various expressions, including how they treat their families and one another.

A more fitting answer that encompasses the overall theme of Malachi would be the concept of covenant loyalty and the consequences of sin, which can be synthesized into the broader theme of faithfulness to God's commandments. This overarching theme speaks to the need for redemption, both individually and collectively, as the people are called to return to right relationship with God, indicating that sin and redemption are critical themes within the text.