Divine standing

Think of this like legal standing. Elturel was essentially deprived of its standing with the gods over the course of fifty years (or more) via a long con perpetuated by Gargauth and various legislative changes.

1. First, the Companion is legally attributed to no god in particular. By doing so, you have created an item of power that is allegedly divine, but which can be attributed to no god.
2. Later, in the name of unity, thanks are offered to the Companion prior to the gods themselves. "First, let us pray to the Companion, whose light watches over us and guides us. Next, to Torm..." And so on. Now the Companion is placed prior to the gods in priority.
3. Eventually, through the pervasiveness of things like the Creed Resolute, people start holding the Companion on a level to the gods themselves. You can't legislate that common people swear oaths to the Companion, but you can lead a horse to water. "I swear to the Companion, if this pays off, blah blah blah." If things like this become common enough, you have elevated reverence of the Companion to the same level as the gods, or even greater.

Via metaphysical means and a long-game devilish con, Elturel was distanced from its patron gods so slowly that when it was stolen from the Prime Material Plane, they hardly noticed -- and did not have the divine standing to storm Hell to take it back, leaving it up to the work of mortal heroes.