They that trust in their wealth - The first reason why there was no cause of alarm is drawn Psalms 49:6-10 from the “powerlessness” of wealth, as illustrated by the fact that it can do nothing to save life or to prevent death. He refers to those who possess it as “trusting” in their wealth, or “relying on” that as the source of their power.
And boast themselves - Pride themselves; or feel conscious of safety and strength because they are rich. It is the “power” which wealth is supposed to confer, that is alluded to here.
In the multitude of their riches - The abundance of their wealth.