Jürgen Moltmann's definition of Original Sin
Jürgen Moltmann defines Original sin as the hubris and presumption of humanity to be like God, but also humanity's resignation and laziness to not be like God's image on earth. Christian theology has traditionally defined Original Sin as the rebellion of man against God, but this is a one sided definition, that can be used to oppress liberation theology and to conserve the political status quo by running out the rebels and freedom fighters. In this post, I'll provide Moltmann's definition of Original Sin from his Introduction to Christian Theology lectures and provide some analysis of it.
Moltmann defines Original Sin in this lecture as follows:
Our definition of original sin therefore must be: 1) hybris and presumption to be as God, 2) resignation and laziness not to be like God's image on earth. Presumption and resignation are the roots of all our actual sins. To put it another way: the sin against the first commandment is the source of all the other sins against the other commandments of God. It is presumption not to acknowledge the Lordship of God. It is resignation not to enter into the "exodus out of Egypt" and to refuse the freedom God has given us. 
Original Sin's traditional definition as Rebellion
Moltmann says that Church tradition has defined Original Sin as "man's rebellion against God" and based the story of the serpent's encounter with Adam and Eve in Eden (Genesis 3). In this story of the Fall, humanity receives two promises through the voices of God and the serpent: the first is that humanity was created as "an image after God's own likeness" and the second is that humanity "will be like God, knowing good and evil (and capable of anything)".
Moltmann describes these promises as follows:
The first definition of sin is well-known: man's rebellion against God as it is depicted in the story of the serpent in Paradise and the sinful fall of Adam and Eve. There was the promise of God, "God created man in his own image, in the image of God he created him," that is, an image after God's own likeness. And then another promise occurred to man in the voice of the serpent, "God knows that when you eat of the fruits of the tree your eyes will be opened and you will be like God, knowing good and evil" (that is capable of everything) [Gen 3:4]. Adam and Eve ate of this fruit and their eyes were opened. They felt ashamed of their nakedness, and they hid themselves from each other and from God. 
According to Moltmann, Church Tradition has used this story to call Original Sin the "hubris and presumption of humanity", because humanity has abandoned its promise to be "like God" and has aspired to sit upon the throne of God. Man seeks to deify himself, and in act of rebellion, like the mythological Prometheus, who stole the divine fire. So humanity exists as a rebel in permanent revolt against God, instead of becoming the image of God on earth. Moltmann does not believe the image of God is in immutable attribute of humanity, because when humanity no longer lives according to the Imago Dei, humanity ceases to be human, and becomes an animal. Moltmann expresses this colorfully when he says "Humanity without acknowledgement of the divine is bestiality".
Moltmann summaries the traditional Christian definition of Original Sin as rebellion in the following way:
According to this poignant story, man's sin can be defined as his godless wish to be like God. Thus the Church tradition called the original sin hubris, the presumption of man to be God himself and to be without a Lord, to be his own master. Original Sin means here the origin of all sins or sin in its origin. Man leaves his human condition and pretends to sit on the throne of God. The deification of man, this is the original sin. Man in his experienceable existence is a rebel against God. His life is nothing short of a permanent revolt against God's lordship. He wants to be divine, and for this reason he becomes inhuman, in fact, a beast. Humanity without acknowledgement of divinity is bestiality, runs an old sentence. For Christian tradition, then, Prometheus, who has stolen the divine fire, is the prototype of the rebellious sinner. 
Criticism of Original Sin defined as "rebellion and revolt"
Moltmann believes that Original Sin is more than "rebellion and revolt", and believes this is a one sided definition that has been abused to preserve societal and political powers in the Church. Moltmann says that political establishments have used this one-sided definition of Original Sin to suppress freedom and justice. Anyone who challenges the traditional political establishment is labeled as "children of protest" and opposed as "rebels" that need to be put out of the Church. Anyone who seeks to establish freedom and justice in the Church, are condemned by the Church as sinful and godless rebels.
Moltmann explains why defining Original Sin as Rebellion is a one-sided danger as follows:
But it seems to me very one-sided to define the sin of the man only in terms of rebellion and revolt. This can easily be used by political establishments to condemn as sinful and godless all revolutions and rebellions for greater freedom and justice. This definition of sin as rebellion against the authority of God and against those who claim to be authorized by God drove the revolutionaries, the rebels and the "children of protest" out of the Church. The Church became with this definition of sin a merely conservative institution associated always with conservative political and societal powers. 
Destiny of man is to be "like God"
Moltmann's definition of Original Sin is linked to his definition of the Imago Dei. The Image of God refer to humanity's destiny to be "like God" or exist in the "likeness of God". Moltmann explains Original Sin as humanity's failure to be "like God" and also as humanity's desire to "not be like God" as well. It's a two sided definition, the includes humanity's rebellion and sloth. Original Sin therefore is humanity's pursuit of the serpent's alternate way of being, that is another likeness other than the "likeness of God", and it is humanity's presumption and resignation to exist in the "likeness of God", as humanity no longer wishes to be like God.
Moltmann explains the link between the Imago Dei and Original Sin as follows:
The expression "like God" or "likeness of God" occurs not only in the voice of the serpent, but first, as we have seen, in the creative voice of God himself. It is the destination of man as created in the image of God to be "like God." Therefore the sin of man is obviously that he followed the voice of another being; specifically, he allowed the serpent to define his destiny. But his sin against God can also be seen in the fact that he no longer wanted to be "like God" and the image of God. Not only the wish to be like God, but also the wish to be no longer like God, is the original sin of man (origin of man's sin). Man forsakes his situation of being the image of God in presumption as well as in resignation. 
The Deadly Sin of Sloth
Moltmann explains that humanity's presumption and resignation to be the Image of God on Earth is identical to the deadly sign of Sloth. Sloth is one of the traditional Seven Deadly Sins, and is also known by the terms tristesse or acedia. Sloth is a state of melancholy sadness the decays humanity when it no longer desires to be like God. Moltmann describes sloth as "hopelessness, inertia, melancholy" and "the seed of sweet decay". Moltmann explains the seriousness of sloth by equating it with the "fearful" and "cowardly" mentioned as destined for Eternal Death in Rev 21:8, and notes they are listed before the murders and other grievous sinners.
Moltmann explains why sloth "is the sin which most profoundly threatens the believer" in his lectures as follows:
This is the other side of the coin of original sin: hopelessness, resignation, inertia, melancholy. From this arises the tristesse and frustration which fill all living things with the seed of sweet decay. Among the sinners whose future is eternal death, the "fearful" or "cowardly" are mentioned in Revelation 21:8 before the unbelievers, murders, and the rest. Temptation then consists not so much in the titanic desire to be like God, but in weakness, timidity, weariness, and in not wanting to be like God or to be what God requires of us. God has exalted man and given him the prospect of a life that is open and free, but man hangs back and lets himself down. 
Original Sin as an act of Commission and Omission
Moltmann summarizes his definition of Original Sin as both an act of commission and omission. Humanity not only acts in rebellion against his destiny to be "likeness of God" but man also fails to aspire to the "likeness of God" as well. Moltmann's two fold definition of Original Sin is a clarifying improvement upon the one-sided definition as "rebellion".
Here is a concluding remark from Moltmann on humanity's two fold act of commission and omission in Original Sin:
God promises the new creation of all things in righteousness and peace, but acts as if everything were as before, as if nothing had changed. God honors him with his promises and calls, but man does not believe himself capable of what is required of him. This is the sin which most profoundly threatens the believer. It is not the evil he does, but the good he does not do, not his sins of commission, but of omission, that accuse him. They accuse him of a lack of hope. For these so-called sins of omission all have their origin in hopelessness and weakness of faith. "It is not so much sin that plunges us into disaster, but rather despair," said the Church Father Chrysostomos. That is why the Middle Ages reckoned acedia or tristitia among the sins against the Holy Spirit which lead to death. 
[^Header Image 1] By Gustave Moreau - The Yorck Project: 10.000 Meisterwerke der Malerei. DVD-ROM, 2002. ISBN 3936122202. Distributed by DIRECTMEDIA Publishing GmbH., Public Domain, Link
[^Header Image 2] By Pieter Brueghel the Elder - Kupferstich, 22,5 × 29,5 cm, Herrausgeber: Hieronymus Cock. Bibliothèque Royale, Cabinet Estampes, Brüssel. Online: zeno.org (Volltextsuche), Public Domain, Link
[^1]Moltmann, Jürgen. An Introduction to Christian Theology. Ed. Douglas Meeks. N.p.: Duke, 1968. 251-4. Print.
Related: acedia, An Introduction to Christian Theology, Image of GOd, imago Dei, Jürgen Moltmann, Original Sin, rebellion, seven deadly sins, sloth, tristitia