Francis Bacon, the English philosopher and statesman. In his Meditationes Sacrae (Sacred Meditations, 1597), in the meditation “Of Heresies”, he wrote in Latin: “Nam et ipsa scientia potestas est” — “for knowledge itself is power.” The three-word English motto is a shortening of that sentence.
What it means
Knowing things gives you the ability to act on them. Bacon's sentence comes from a religious meditation, but it has become a general claim that understanding is a form of strength.
Where it comes from
Wikiquote gives the Latin as “Nam et ipsa scientia potestas est”, translated “For knowledge itself is power”, from the meditation “De Hæresibus” (Of Heresies) in Meditationes Sacrae (1597), a short book of religious meditations. The sentence belongs to an argument in that meditation, not to a book about education.
Shorter Latin forms, “scientia potentia est” and “scientia est potentia”, are also quoted, and Wikiquote lists them as variants. The bare English “Knowledge is power” is a shortening of Bacon's sentence, which says that knowledge itself is power.