そんな話はなし、簡単かんたんに信しんじ込こまないでよ。

sonna hanashi, kantan ni shinjikomanaide yo.

Don't just naively believe a story like that. casual

彼かれはその情報じょうほうが真実しんじつだと信しんじ込こんでいました。

kare wa sono jouhou ga shinjitsu da to shinjikonde imashita.

He was firmly convinced that the information was true. neutral