“Blessed are the pure of heart for they shall see God.” (Mt 5.8) These are the words of Jesus as recorded in a set of teachings known today as the Sermon on the Mount.
Although some may hear this as “Be good and when you die you will go to heaven,” there is a much deeper and more profound teaching here that is very relevant for our life today. So let’s unpack this statement.
The “heart” has long been a metaphor for the subconscious and our feeling nature. The heart is the seat of desire, emotion, intuition and love. In contrast to this is the conscious and rational function of the mind.
Why did he not say “Blessed are the pure of mind or body….” rather than the “pure of heart”? Why is it we can “see God” only with a pure heart?
And, what does it mean to “see God”?
Addressing this last question, we have a quote from Meister Eckhart, a twelfth century German mystic.
“The eye through which I see God is the same eye through which God sees me; my eye and God's eye are one eye, one seeing, one knowing, one love.” 
I “see God” by letting God see through me; God sees through me only when my heart is pure.
What does it mean to have “a pure heart”? Pure means “uncontaminated, untainted, clear and fresh.”
It would appear that infants are born with a pure heart; and in the words of the poet William Wordsworth, they are “trailing clouds of glory…” This glory seems to follow us for a short time into childhood before it fades away as we becomes adults.
THERE was a time when meadow, grove, and stream,
The earth, and every common sight,
To me did seem
Apparell'd in celestial light,
The glory and the freshness of a dream.
It is not now as it hath been of yore;—
Turn wheresoe'er I may,
By night or day,
The things which I have seen
I now can see no more.
Imagine a spring in a high meadow that yields fresh, pure, crystal-clear water. This flowing water becomes a running brook, and then a large stream; and then a wide river flowing down the mountain and through farmlands whose runoff dumps pesticides, hormones and antibiotics into it; and past industrial cities that pump sewage and chemical contaminants into it. That once pure river may become so contaminated that it no longer supports life. (I once lived near a river that was so polluted it actually caught on fire!)
The way we purify a river is to simply stop dumping pollutants into it; as we cease to contaminate the river it will purify itself.
And so it is with the heart. The heart becomes pure when we cease to contamination it with toxins from the ego. The heart is polluted by greed, by hatred and by fear. These forces adulterate the heart and reinforce the ego.
Most of our contamination is hidden from awareness. To become free of toxins we must become aware of their existence; aware of their existence we can cease holding on to them.
Ego identification dumps the toxins of greed, hatred and fear into the heart. Letting go of ego identification purifies the heart.
To cease identifying with the ego is to clarify our perception: we see things as they really are rather than as a mirror of our own shadow. To be free from ego identification is to have a pure heart. The pure heart sees God in all things.
“If the doors of perception were cleansed every thing would appear to man as it is, Infinite.
 Sermons of Meister Eckhart
 William Wordsworth. Ode to Intimations of Immortality.
 William Blake. Marriage of Heaven and Earth.