In his Essay Concerning Human Understanding he argued that human beings are not born with innate ideas divinely implanted in their minds. Rather, the human mind at birth is a blank slate upon which are imprinted sensations derived from contact with sense data. These sensations, combined and rearranged by the mind, are the source of ideas. Therefore, knowledge is derived from experience or empirical data. And if there are no innate ideas, then human beings are not born with original sin, contrary to Orthodox Christianity.
Evil is the result of poor environment, lack of training, and misinformation.
Earlier writers such as Chillingworth had argued that human understanding was limited, Locke tries to determine what those limits are. We can, he thinks, know with certainty that God exists. We can also know about morality with the same precision we know about mathematics, because we are the creators of moral and political ideas. In regard to natural substances we can know only the appearances and not the underlying realities which produce those appearances. An excellent and brief introduction to Locke's thought can be found at John Locke .