One of the things that has most struck me every time is Malachi 2:10-16. Aside from the very difficult Hebrew of vv. 15-16, it has a fascinating train of thought.
1. The people are criticised for marrying outside the covenant community (v. 11), and divorcing wives within the covenant community (v. 14).
[I think this is literal intermarriage, not a metaphor for unfaithfulness to God because:
a) In v. 14, Yahweh is the witness between you and the wife of your youth. He is a judicial third party, not one of the two parties in the marriage.
b) While marriage is a common OT picture of Yahweh’s relationship with Israel, he is (always?) the husband, Israel the wife. Here Judah is the husband.
c) There is no other indication in Malachi that formal idolatry is a problem, eg., Baal-worship in the monarchy. Rather the problem is empty formalism.
d) Pagan intermarriage is a major contemporary problem in Ezra 9-10 and Nehemiah 13:23ff.
2. Other than the biblical theology problem that divorce misrepresents Jesus' steadfast love for his church, the main problem with this intermarriage and divorce is that the purpose of marriage is to seek godly offspring (v. 15). So, at least in Malachi, it's not about companionship, or refraining from sexual immorality, but about populating the land with godly children. There is an implicit assumption here that the children of covenant members will themselves be godly.
3. The point of godly offspring, istm, is to contribute to God's program of 1:5, 1:11, 1:14, 3:12, 3:17-18. The hope of Malachi is that a repentant, godly, chosen people in the land will make the nations sit up and take notice, see how wonderful it is to serve Yahweh, and how awful it is to despise him, and so
"My name will be great among the nations, from the rising to the setting of the sun. In every place incense and pure offerings will be brought to my name, because my name will be great among the nations ... For I am a great king ... and my name is to be feared among the nations." 1:11, 14.
Malachi too was postmill!