This paper aims at uncovering and fragmenting the foundations of Locke’s theory of knowledge as addressed in An Essay Concerning Human Understanding. Systematically, it reveals the origin of knowledge, develops and evaluates the limits of human knowledge and the extent of understanding, as well as critically examines what is knowable with more emphasis on certain and probabilistic grounds. Certainty is attainable through two principal means, comprising the use of the human senses and adoption of abstract reasoning. The sources of human knowledge are significant in easing and enhancing our knowledge of God as it facilitates the justification of his existence, portraying orderliness in the world, and positing mankind as part of God’s creatures expected to implement moral lifestyles through the appeal to reasoning as a law of nature even when in a state of nature. The value of this principle is evident for self-preservation, including an essential moral obligation for the preservation of everybody. The ethical basis of the state of nature featuring both moral and natural laws, signals the natural moral nature of man as determined by a rational order designed by a rational and knowable God.