Why did God allow evil and the devil to exist?

The Orthodox nuance

It must honestly be said from the start: Scripture does not give a detailed account of the fall of the angels, as it does for the fall of Adam, but rather allusions, read together by Tradition. Isaiah speaks of the fall of a "shining star" from heaven (Isaiah 14:12-15), in a text addressed first to the king of Babylon, and Ezekiel has a similar word about the king of Tyre (Ezekiel 28:12-17); the Fathers read these passages also as windows onto the fall of a proud angel. Christ Himself says "I saw Satan fall like lightning from heaven" (Luke 10:18), and Revelation speaks of a war in heaven, in which Michael and his angels triumph over the dragon (Revelation 12:7-9). The Epistle of Jude and the second of Peter mention angels who did not keep their proper rank and were cast down.

From this whole fabric of texts, Tradition has kept a clear teaching at its core, even if not in detail: the angels were created good, with real freedom, like man, and one of them, the most brilliant, chose pride, wanting God's glory without submission to God, and in that choice appeared what we call evil.

Here is a very important point, especially clarified by Saint Maximus the Confessor: evil has no being of its own, it is not a second principle standing opposite the good, as in a dualism. It is rather like blindness, which is not a thing in itself, but the absence of sight, or like rust, which does not exist apart from iron, but is the iron itself spoiled. So it is with evil, a good will, cut off from its source, turned back on itself. That is why the question "why did God create evil" starts from a mistaken assumption: He created only good natures, and evil appeared in the way a free nature, first angelic, then human, misused the freedom given as a gift.

The question remains why God allowed this freedom to exist, with all its risk. The answer, already touched on regarding free will, is that a freedom that could never say no to God would not be true freedom, but a mechanism, and a forced love would no longer be love. In Orthodoxy, the devil is considered real, not a symbol for the general evil of the world, but also already decisively defeated through Christ's death and Resurrection, even though he continues to work until the end of the ages. That is why Baptism includes prayers of renunciation of Satan, and ascetic literature speaks much of the "unseen warfare," but always with eyes fixed on the victorious Christ, not on endless fear — "greater is He who is in you than he who is in the world" (1 John 4:4).