Purity of souls comes from contact with Lord Shiva

ज्ञानोपदेशो नामैकत्रिंशोऽध्यायः

Knowledge advice Chapter 31

वायुरुवाच |

Vayudeva said (to the sages)

इति नाग्निरसौ दुष्येत्ताम्रसंसर्गकारणात् |
नाग्नेरशुचिसंसर्गादशुचित्वमपेक्षते || 44 ||
अशुचेस्त्वग्निसंयोगाच्छुचित्वमपि जायते |
एवं शोध्यात्मसंसर्गान्न ह्यशुद्धः शिवो भवेत् || 45 ||
शिवसंसर्गतस्त्वेष शोध्यात्मैव हि शुध्यति |

In this way the fire does not get polluted/corrupted because of the contact of copper. Fire does not get impure by the contact with impure (things).
But purity arises in the impure by the contact with fire. In this (same) way, Lord Shiva does not become impure by contact with a soul which needs to be purified. Only this soul which needs to be purified get purified by contact with Lord Shiva.