godan verb (-mu), transitive verb
believe firmly; believe implicitly
Expresses a strong, often unquestioning belief; the person is completely convinced, sometimes to the point of being misled.
彼はその噂をすっかり信じ込んでしまった。
He completely believed that rumor.
彼女は彼の嘘を真実だと信じ
She firmly believes his lies are the truth.