John Winthrop was the founder of the Massachusetts Bay Colony in the 1600's. A skilled orator and philosopher, he coined the idea of moral liberty in contrast to natural liberty.

Winthrop believed there were two kinds of liberty in this world and that man was entitled to liberty. Natural liberty is the most straightforward of the concepts—it is the freedom to do as one chooses. Natural liberty is constrained by the laws of nature and nothing else. In that way, man is free to do anything he can physically accomplish, no matter the consequences. People have the natural liberty to jump off a cliff, but that doesn't mean they are free from the burden of the consequences.

Moral liberty was Winthrop's brainchild and the concept he championed for peace in his colony. He believed that the church and government overtook human decision making and superseded them, implementing moral liberty or civil liberty as a way to distinguish right from wrong. Moral liberty states that people are free to take actions so long as they are good and honorable. This would prevent actions that harm others or themselves and actions that are destructive in nature. Moral liberty is less a right than an obligation to do good.

In what is known as John Winthrop's "Little Speech on Liberty" (1645), Winthrop, a founder of the Massachusetts Bay Colony, outlined in a court proceeding his views on liberty.  Specifically, he argued that

There is a two-fold liberty, natural (I mean as our nature is now corrupt) and civil or federal [moral liberty].  The first is common to man with beasts and other creatures.

Natural liberty, according to Winthrop and his Puritan belief system, simply meant that man, like beasts, has liberty to do whatever he wants to do--good or evil.  In the Puritan belief system, mankind is inherently evil and corrupt, the result of the doctrine of Original Sin.  That is, all men are descendants of Adam, the original sinner, and as such, are destined to go to Hell unless they are saved by God's grace.  Winthrop said that natural liberty "makes men grow more evil, and in time to be worse than brute beasts."  In other words, the exercise of natural liberty results in increasingly evil behavior.

Civil (or moral) liberty, however, is the

. . . proper end and object of authority. . . and it is a liberty to that only which is good, just, and honest.

In other words, moral liberty, because it is imposed by the government and church, replaces natural liberty with rules that foster proper behavior.  One could argue, of course, that "liberty" imposed by government and the church is not freedom but restriction, but if we keep in mind that the Puritans believed that man was naturally evil, we can see the logic in replacing that natural and evil liberty with civil or moral liberty (meaning, Puritan authority) that leads men to do good rather than evil.  Moral liberty is assumed to be good because it is the result of an adherence to Christian (Puritan) principles.

In sum, then, natural liberty leads to evil because men are inherently evil; moral liberty fights natural evil because it is founded on a pure Christian belief system and the authority of the church and government, which are, for practical purposes, inseparable.