Sufism looks at man as a heart and a spirit and a secret, and the secret is the deepest part. The secret of man is God's secret; therefore it is in God. My secret is God's innermost knowledge of me, which He alone possesses. It is God's secret knowledge of myself in Him, which is a beautiful concept. The heart is the faculty by which man knows God and there Sufism develops the heart.
This is a very important concept in the contemplative life, both in Sufism and in Christian tradition. To develop a heart that knows God, not just a heart that loves God, but a heart that knows God. How does one know God in the heart? By praying in the heart. The Sufis have ways of learning to pray so that you are really praying in the heart, from the heart, not just saying words, not just thinking good thoughts or making intentions or acts of the will, but from the heart. This is a very ancient Biblical concept that is carried over from Jewish thought into monasticism. It is the spirit which loves God, in Sufism. The spirit is almost the same word as the Biblical word "spirit" -- the breath of life. So man knows God with his heart, but loves God with his life. It is your living self that is an act of constant love for God and this inmost secret of man is that by which he contemplates God, it is the secret of man in God himself.
-- Thomas Merton,
speaking to a group of Catholic sisters in Alaska,
2 1/2 months before his death in 1968
with thanks to louie,louie