The Shia start knowing Imam Hussein peace be upon him before their birth. In other words, at the time of creation, Allah Almighty mixed their existential soil with the knowledge of Imam Hussein (AS) and made this knowledge take root in their souls. Therefore, the Prophet (pbuh) says:
«إِنَّ لِلْحُسَينِ(ع) فی بَواطِنِ المُؤمِنینَ مَعْرِفَةٌ مَکْتُومَةٌ» ؛
The knowledge of the believers about Imam Hussein (AS) has infiltrated in their hearts.
According to this narration, the knowledge, love and inner love of the Shias about Imam Hussein peace be upon him is an innate and inborn issue.
source : erfan.ir