Purification of Heart

Indeed, all sins poison the heart and lead to its illness, disease and destruction. Being in effect as harmful to the heart as actual poisons are to the body, they sicken it and define its will as other than what God wants (of His servant). Imam Ibn Al-Mubarak, may Allah have mercy on him, said: 

Knowing the heart and its qualities and functions is an integral part of the religion and a basis for those who seek the cause of Allah

Before you can fill any vessel, you must first empty it. The heart is a vessel. And like any vessel, the heart too must be emptied – before it can be filled. One can never hope to fill the heart with God, so long as that vessel is full of other than Him. To empty