Once the prophet (s.a.w) said:"indeed there is a piece of flesh in the body ,if it is sound , the entire body is sound , when it is corrupt , the entire body is corrupt . Truly it is the heart "
The heart is a place of belief and piety , or disbelief , hypocrisy and polytheism.
The prophet(s.a.w) said:"Piety is right here and he pointed st his heart breast three times"
IMAN is belief , speech and action.
Belief and conviction in the heart , speech of the tongue and the action of the limbs . The heart believes and affirms which generates the saying of shahadah (testimony) on the tongue then the heart acts with love ,fear and hope. After the tongue remembers Allah and recites Quran and limbs prostrate , bow and perform actions that bring one closer to Allah.The body follows the heart therefore nothing enters the heart except that it necessitates is shown on the body in somebody.