to treat one another with absolute sincerity (idiom) / to show total devotion
推心置腹
tuī xīn zhì fù
to give one's bare heart into sb else's keeping (idiom) / sb has one's absolute confidence / to trust completely / to confide in sb with entire sincerity
坦诚相见
tǎn chéng xiāng jiàn
to trust one another fully / to treat sb with sincerity