The world, the flesh, and the devil
In Christian theology, the world, the flesh, and the devil (Latin: mundus, caro, et diabolus; Greek ό κοσμος, ή σαρξ, και ό διαβολος) are often traditionally described as the three enemies of the soul. As the sources of temptation, they are sometimes opposed to the Trinity.
The roots of this triad are possibly to be found in Jesus' parable of the Sower: the three scenes of unproductive soil represent "Satan" (birds eating the seed), shallow and unreceptive believers (corresponding to weak "flesh"?), and "the cares of the world and the lure of wealth" (Gospel of Mark 4:15–17). These three are also present as a triad in the Letter to the Ephesians chapter 2, verses 1–3: "You were dead through the trespasses and sins in which you once lived, following the course of this world, following the ruler of the power of the air, the spirit that is now at work among those who are disobedient. All of us once lived among them in the passions of our flesh, following the desires of flesh and senses..."
Many Christian sources refer to the world, the flesh, and the devil.
Peter Abelard states in his Expositiones that
Tria autem sunt quae nos tentant, caro, mundus, diabolus.
[1] (There are three things which tempt us, the flesh, the world, and the devil.)
Thomas Aquinas refers to the world, the flesh, and the devil in the Summa Theologica.
The Council of Trent sixth session, degree on justification, [2]
Nevertheless, let those who think themselves to stand, take heed lest they fall, and, with fear and trembling work out their salvation, in labours, in watchings, in almsdeeds, in prayers and oblations, in fastings and chastity: for, knowing that they are born again unto a hope of glory, but not as yet unto glory, they ought to fear for the combat which yet remains with the flesh, with the world, with the devil, wherein they cannot be victorious, unless they be with God's grace, obedient to the Apostle, who says; We are debtors, not to the flesh, to live according to the flesh; for if you live according to the flesh, you shall die; but if by the spirit you mortify the deeds of the flesh, you shall live.
The phrase may have entered popular use in English through the Book of Common Prayer, which includes in its Litany
[... F]rom al the deceytes of the worlde, the fleshe, and the deuill: Good lorde deliuer us.
or, in modern editions,
[... F]rom all the deceits of the world, the flesh, and the devil, Good Lord, deliver us.
John of the Cross cites the world, the flesh, and the devil as threats to the perfection of the soul, and offers different "precautions" to be taken against each of these.
Some have responded to the idea of temptation by teaching or practicing asceticism; see also ascetical theology and mortification of the flesh. The question of whether the world and the flesh are inherently bad and what the individual's proper relationship to them ought to be has long been debated in many philosophical and spiritual traditions; see Manichaeism and Gnosticism.