I think this phrase may go back to the 19th/early 20th century writer Marie Corelli.

This from an essay about Corelli makes the phrase more clear:

"Halfway through the essay, Corelli argues that only those peoples whose culture shows that they are endowed with the gift of imagination, are ‘conscious of the Highest source of all creation’ (276). Quoting a passage from Wordsworth’s ‘Ode: Intimations of Immortality from Recollections of Early Childhood’, with which Corelli seemingly substantiates her insistence on the connection between man, his divine nature and God who gives man a soul, she now goes on to attack materialism as the opposite of imagination."
http://www.victorianweb.org/authors/corelli/kuehn1.html

Highest, in this sense, means at the highest level or universal level as opposed to a lower or more localized level.

I think source only makes sense in the full phrase "source of creation" - again not in a local sense of creation, but in the sense of the first creation.

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Note added at 4 days (2012-05-01 14:09:19 GMT)
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Even if the author of this article had not read Corelli's book, it is through books such as Corelli's that certain expressions, and their meaning, become part of the language. Sometimes these phrases are only known as part of the language of the subject area and sometimes they become a more general part of the language as a whole.