The heart is a temple that has been placed by God in everyone, a temple that houses the Divine spark within us. In a saying much beloved by the Sufis, God reveals, "I, who cannot fit into all the heavens and earths, fit in the heart of the sincere believer." Therefore, this temple within each of us is more precious than the holiest shrines and temples on earth. The earthly temples were built by great saints and prophets, but the temple of the heart was built by God to house God.
Many of us have neglected our heart temples. We have also allowed in our hearts the worship of idols. By idols I mean the ephemeral things of this world. We have worshipped worldly success fame, money, and power and devoted ourselves to hungering for them and pursuing them. Most of us have spent far more time on these worldly goals than we have in seeking God or in seeking personal transformation. One of the fundamental practices of Sufism is to repeat the phrase, la ilaha illallah, "There are no gods but God." One level of meaning of this sacred phrase is, "There is nothing worthy of worship but God." The discipline of Sufism includes cleansing our hearts of the idols we have enshrined there, making them suitable temples for God's presence.
My Sufi master taught that the goal of Sufism is to develop hearts that can pray. Anyone can learn the outer forms of worship, but it is much harder to teach our hearts to pray. The outside is always easier than the inside. For example, it is not difficult to make our outsides clean by bathing and putting on clean clothing, but it can be very difficult to cleanse our insides. Through sincere, patient practice of worship, service, and other spiritual exercises, the heart becomes cleansed and expanded.
In the kundalini tradition, the heart is often described as the mediator between the three lower chakras and the three upper chakras. If the heart is opened, energy will not remain stuck in the lower chakras. The heart helps draw the energy upward, activating the upper, more spiritual chakras.
In the Sufi tradition, the heart is seen as a mediator between the outer influences of the world and the spiritual influences within us. If our pride, greed, and other negative tendencies become involved with things of the world, they put out a kind of heat and smoke that distracts us and hides the spiritual light of the heart. The more we open to that inner light, the more we can see clearly our own negative tendencies, and also, the more we strengthen our positive and spiritual tendencies.