Strictly speaking, 'fountain of all knowledge' is correct rather than 'fount' or 'font'. All three are used nowadays, but the original �fountain' version was a 17th century quote and the other two are just more modern variants of that original.
The quote is from 'An Essay Concerning Human Understanding' by the philosopher, John Locke, published in 1690. Clearly, in it he was referring to God.
"Reason is natural revelation, whereby the eternal father of light, and fountain of all knowledge, communicates to mankind that portion of truth which he has laid within the reach of their natural facilities."
In a 14th century book, God was referred to as "the fountain of all goodness" and in the 16th century Book of Common Prayer there is a reference to "Almighty God, the fountain of all wisdom".
However, Locke's quote - given above - is the earliest instance of the concept of knowledge having a �fountain' as its source.