They have purified their hearts from other than Allah and they have made them as pathways for rivers to run, receiving knowledge of the Divine Presence.

Imam Ghazali said in the honour of Awliya (Friends of Allah), al-Munqidh min ad-Dalal, page 131

The hearts of the Awliya have been cleansed to such an extent that they are able to receive exclusive knowledge of the Creator.