Holiness hidden in the lowest places

ברש"י מקום שהיו ישראל מקולקל ביותר.

In Rashi: the place where Bnei Yisrael were most corrupted.

The Sefas Emes draws on Rashi's comment that the parsha of Kedoshim was given precisely where Bnei Yisrael's conduct had been most broken down - the lowest, most damaged place.

להראות כי בכ"מ שיש הסתר יותר יש שם נקודה קדושה טמונה כי זה המכוון להסיר ההסתר:

This comes to show that in every place where there is greater hester (concealment), there is hidden there a deeper nekudah kedoshah (holy point) - for this is the very purpose: to remove the concealment.

Wherever Hashem's presence seems most hidden, an especially powerful holy spark lies buried beneath the surface. The whole point of the concealment is to be peeled away, revealing the kedushah within - so the lowest places hold the highest potential for sanctity.

Summary: The call to holiness was issued precisely where Bnei Yisrael were most corrupted, teaching that the greater the concealment, the deeper the holy point hidden within - and that the purpose of every concealment is ultimately to be lifted away to reveal it.