I was surprised the other day to find that the Cloud of Unknowing was not available on the internet..
The Cloud of Unknowing is an anomymous work from the late 14th century that is sort of a handbook for a novice entering the contemplative life. The original work has about 40,000 words, but I can summerize it in two sentences.
Seek the face of the Lord.
If any thoughts enter your mind, ignore them.
This prayer probably predates christianity, and is found in every major religion. In the 1960's, the prayer was repackaged and made into a business by three catholic priests and renamed 'Centering Prayer', here's the original 14th century text.