信以为真

The supposed prince was really a beggar in disguise.

那个被信以为真王子, 原来是个乞丐。

You may be joking but she's simple enough to believe you.

你也许是在开玩笑, 但她却愚蠢得信以为真。

He believes everything with childlike simplicity.

他像孩子样单纯, 对切都信以为真。