In Nahum 3, the inner meaning is mostly teaching us about the need to reason from a sound spiritual foundation -- to root out false ideas, adopt true ones, and genuinely live by them.
In Nahum 3:1-4, All those who would like to destroy the truths of the Lord’s church will be gathered in the spiritual world; they gravitate towards each other, and essentially form hellish societies.
This is explained further, in Nahum 3:5-7: In the spiritual world it becomes impossible for evil people to hide their true loves behind a pretense of being good.
In Nahum 3:8-17, the inner meaning is that any doctrinal truths will be forgotten by those people, because they have lost all desire to know them, and certainly all desire to use them as a guide to behavior.
Nahum 3:18 makes the point that human reason isn't a reliable foundation. Assyria stands for the ability to reason, but if it starts from an unsound framework, it's not useful.
Nahum 3:19 hammers the point home: There is nothing of any soundness left in them.