to believe implicitly; to believe firmly; to believe entirely
完全に騙されて信じ込んでいた。
I had been completely taken in and firmly believed it.
誰かに言われたことをそのまま信じ込んでしまった。
I ended up completely believing what someone told me without question.
嘘を真実だと信じ込む傾向がある。
There is a tendency to firmly believe lies as truth.