The Sufferance of Evil
The chief aim and effort of the Lord’s Divine Providence is that a man shall be in what is good and in what is true at the same time; for thereby man is man, since he is then an image of the Lord. But because, in his life in the world, he can be in what is good and in what is false at the same time, and also in what is evil and what is true at the same time, nay, even in evil and at the same time in good, and thus be a double man, as it were, and because this division destroys God’s image and so destroys the man, therefore the Lord’s Divine Providence in all its workings seeks to prevent this division. Furthermore, because it is better for man to be in what is evil and in the same time in what is false than to be in good and at the same time in evil, therefore the Lord permits it; not as one willing it, but as one unable to prevent it consistently with the end, which is salvation.
—Divine Providence, n. 16