Felix culpa is a Latin phrase that comes from the words felix (meaning "happy," "lucky," or "blessed") and culpa (meaning "fault" or "fall"), and in the Catholic tradition is most often translated "happy fault," as in the Paschal Vigil Mass Exsultet O felix culpa quae talem et tantum meruit habere redemptorem, "O happy fault that earned for us so great, so glorious a Redeemer."
The Latin expression felix culpa derives from the writings of St. Augustine regarding the Fall of Man, the source of original sin: “For God judged it better to bring good out of evil than not to permit any evil to exist.” (in Latin: Melius enim iudicavit de malis benefacere, quam mala nulla esse permittere.) The phrase appears in lyric form sung annually in the Exsultet of the Easter Vigil: "O felix culpa quae talem et tantum meruit habere redemptorem," "O happy fault that merited such and so great a Redeemer." The medieval theologian Thomas Aquinas cited this line when he explained how the principle that "God allows evils to happen in order to bring a greater good therefrom" underlies the causal relation between original sin and the Divine Redeemer's Incarnation, thus concluding that a higher state is not inhibited by sin. The Catholic saint Ambrose also speaks of the fortunate ruin of Adam in the Garden of Eden in that his sin brought more good to humanity than if he had stayed perfectly innocent. In the appendix to Leibniz's Theodicy, he answers the objection concerning he who does not choose the best course must lack either power, knowledge, or goodness, and in doing so he refers to the felix culpa.
The concept also occurs in Hebrew tradition in the Exodus of the Israelites from Egypt and is associated with God’s judgment. Although it is not a fall, the thinking goes that without their exile in the desert the Israelites would not have the joy of finding their promised land. With their suffering came the hope of victory and their life restored.
In a literary context, the term "felix culpa" can describe how a series of unfortunate events will eventually lead to a happier outcome. The theological concept is one of the underlying themes of Raphael Carter's science fiction novel The Fortunate Fall; the novel's title derives explicitly from the Latin phrase. It is also the theme of the fifteenth-century English text Adam lay ybounden, of unknown authorship, and it is used in various guises, such as "Foenix culprit" and "phaymix cupplerts" by James Joyce in Finnegans Wake.
In the philosophy of religion, "felix culpa" is considered as a category of theodicy in response to the problem of evil. Although it is usually discussed historically, there are still contemporary philosophers, such as Alvin Plantinga, who defend the felix culpa theodicy.