Why does God allow sin to exist?

Behind everything God does stands a single purpose: His own glory. That is the short answer to why He allows sin. Not that God takes pleasure in evil, and not that His glory somehow depends on it, but that within His sovereign plan He has chosen to let sin run its course for a season, so that His response to it might display who He is in a way the universe would otherwise never have seen.

God did not permit evil to fill a gap in His happiness or to make Himself more glorious than He already was. Whatever purpose sin serves, it adds nothing to God. He was complete without it.

What sin does is make the perfections of God visible to His creatures — perfections that, by their very nature, could only be shown in a world where something had gone wrong.

Consider mercy. Mercy is kindness shown to the guilty; where there is no guilt, there is nothing for mercy to answer. The same is true of grace toward the undeserving, of forgiveness toward the offender, of patience toward the rebel, and of justice against the wrongdoer.

A universe without sin would be a universe that never saw God forgive, never saw Him show mercy to those who had earned wrath, never watched grace triumph over judgment. In permitting sin, God opened the stage on which the deepest riches of His character could be shown. Paul says exactly this: God saves sinners “that in the ages to come He might show the exceeding riches of His grace in His kindness toward us in Christ Jesus” (Ephesians 2:7).

This is why the existence of sin, terrible as it is, sits inside God's purpose rather than outside His control. Allowing sin is not the same as approving it — God never endorses the evil He permits.

He lets sin exist not because He is indifferent to it, but because His triumph over it will reveal Him more fully than an untested innocence ever could. And that revelation is not only for us. Paul tells us that what God is doing is watched from heaven itself: God's purpose is “that now the manifold wisdom of God might be made known by the church to the principalities and powers in the heavenly places” (Ephesians 3:10). The angels themselves learn something of God by watching how He redeems a fallen world.

So sin is permitted, but it is never pointless, and it is never permanent. It serves, for a time, to show the watching universe attributes of God that only a rescued world could reveal — and then it is done away with.