What does Pope mean in the following lines: If plagues or earthquakes break not Heav'n's design, Why then a Borgia, or a Catiline?

Pope's didactic poem, "An Essay on Man", is intended as a complete system of morality written in verse. In the lines quoted, Pope is trying to explain what is sometimes referred to as the problem of "theodicy" or divine justice. This problems revolves around the apparent contradiction between our immediate and direct perception of evil and injustice in the world and the claim of the existence of a benevolent, omniscient, omnipotent God. Pope himself was...

Pope's didactic poem, "An Essay on Man", is intended as a complete system of morality written in verse. In the lines quoted, Pope is trying to explain what is sometimes referred to as the problem of "theodicy" or divine justice. This problems revolves around the apparent contradiction between our immediate and direct perception of evil and injustice in the world and the claim of the existence of a benevolent, omniscient, omnipotent God. Pope himself was a Roman Catholic and in this passage is arguing that acknowledging the existence of human evil is not incompatible with belief in the Christian God. 

In this passage, Pope asserts that what might appear to us the harsh and unforgiving elements of the natural world, and disasters such as earthquakes, droughts, and floods, are part of a larger design known only to God. From our limited point of view, we only see how this design affects us, not the bigger picture.

He next compares extremes of human evil to natural disasters. Just as in the twenty-first century, the person we normally invoke when we want to talk about evil is Hitler, so in Pope's period two of the best known example of evil would have been the Cataline and the Borgia family.

The Catiline is Lucius Sergius Catilina, who attempted to overthrow the Roman Republic. He is the target of Cicero's Catilinarian Orations, which were widely read in English schools in Pope's period and would have been familiar to most of his readers. The House of Borgia was a famous (or infamous) Renaissance Italian family, known for unscrupulous behavior. 

Thus in these lines Pope is suggesting that human evil is no more incompatible with the existence of a benevolent God than natural disasters. Eventually, he argues that as we cannot presume to know what goes on in the mind of God:

Know then thyself, presume not God to scan

The proper study of Mankind is Man