Because thunder is now scientifically explained, people no longer believe it's the wrath of god against humans. So thunder isn't as frightening as it used to be.
Literal
Thunder [topic-は] scientifically is-being-explained [because-から], thunder [subject-が] human [against-に対する] god [possessive-の] wrath is [quotative-など-と]-as-for people [topic-は] anymore are-not-believing. Therefore, thunder [also-も] before [as-much-as-ほど] frightening thing has-not-become.
A long, multi-clause explanatory sentence stacking several patterns. に対する ('against / toward') is the attributive form of に対して. などと marks 'such things as,' framing the quoted belief dismissively. ~なくなってきている = 'has come to not be / is becoming not' — ~なくなる ('come to not be') + ~てくる ('change over time') + ~ている (ongoing). A textbook-style passage worth a careful pass.