Akademik

П-585
(О) СВЯТАЯ ПРОСТОТА! Ilnterj these forms only often used when responding to the interlocutor's statement, suggestion etc) how naive ycu are (he is etc): (oh,) blessed innocence oh holy simplicity!
«Значит, надо обязательно обманывать кого-то?» - «О святая простота!» - говорит Юрка... (Михайловская 1). "Does that mean that deception is necessary?" "Oh, blessed innocence," says Yuri .. (1a)
The Russian phrase is a loan translation of the Latin sancta simplicitas. The use of the Latin phrase in this meaning is attributed to Jan Hus (John Huss), a Bohemian religious reformer (13707-1415)

Большой русско-английский фразеологический словарь. — М.: ACT-ПРЕСС КНИГА. . 2004.