Why must が be used here?

[fuubuu]

ほどなくして神様達はこの下界に住み付いた。多くの神様達が永住することを決めたらしい。

Why does が have to be used in that sentence? What difference would it have if は was used instead? Thanks.

[community.japanese]

fuubuu さん、
こんにちは。
It doesn’t have to be が.
は is ok here grammatically.
I think the author wanted to emphasize “the many gods”here.

Yuki　由紀
Team JapanesePod101.com