He doesn't explain unless you ask him.

Roughly two weeks later, in the middle of August or best described the hottest few weeks of the year, they got a new ally down here at the Town. Of course before it there were some debates on certain cases.

For example, the Grand Masters insisted that whatever Eric saw must have been an illusion from opening his eyes once deep in Lava. At the same time, Eric had a good debate against it. For example, "How could you know that if you never swam in Lava?"

The debate didn't last long, since after all this Volcano is in other terms hand made. Grand Masters of the ancient times formed it, or best described the first Calidum Lutum send down by the God "Calidus".

If there was something to see down there, they would have seen it. In any case Eric could object religious claims since most tend to be untrue, he even has facts against them. But the religion here? It sounds the most believable.