As we discussed last time, doubt is rooted not in truth, but in suggestion.

To be sure, the devil baits the hook with a piece of truth, but it’s always a half-truth. Indeed, he often quotes the word of God itself (as he did with Jesus), but leaves out the full picture. So, for instance, he recites a bit of Psalm 91 to our Lord:

“Then the devil took him to the holy city, and set him on the pinnacle of the temple, and said to him, ‘If you are the Son of God, throw yourself down; for it is written, “He will give his angels charge of you” and “On their hands they will bear you up, lest you strike your foot against a stone’” (Matthew 4:5-6).

Note that if. It’s not a frontal assault, but a poisonous hiss. He suggests that Jesus is not the Son of God, that he has to prove it, that he should doubt it, that he should take matters into his own hands and not trust his Father.

It’s the same trick he pulled in the Garden of Eden: Get the mark to buy into a lie. In the case of Adam and Eve, Satan tried (and succeeded) to get them to buy the lie that they were gods. In the case of Jesus, he tried (and failed) to get him to buy the lie that he is not God.

The devil’s work of suggestion is like the encouragement of an infection in our thinking. Just as our bodies are surrounded by germs in our air, food and water, so we encounter ideas all the time, and, as a general rule, our mental immune system can digest what is useful and fight off what is harmful.

We are subjected to people babbling ideas all day long from the media, work, family, school, church, etc. Most of what is said doesn’t affect us too powerfully. Can you remember what you talked about at lunch three weeks ago? Do you remember headlines in the paper on June 7, 2009? What was the homily about at Mass three months ago?

Most information gets filtered, some gets digested (that’s the process we call “learning”), and some of it, now and then, becomes inflamed.

Doubt is the inflammation of the mind and spirit, rather like an infection is an inflammation of the body. The devil figures that if he can inflame some idea as a sort of spiritual cancer, he can turn your own mind against you and make your reason the enemy of your sanity. As G.K. Chesterton observed, “The madman is not the man who has lost his reason. The madman is the man who has lost everything except his reason.”

The fever of doubt occurs when a difficulty becomes, as it were, inflamed. We become trapped in the prison cell of a single idea: “If Jesus is God, why did he not know when the world would end? If God is love, why is the Old Testament so violent? If the Catholic Church is true, why all the corruption?” etc.

We let ourselves become consumed with the fear that some difficulty we face (it can be virtually anything) is not merely a difficulty, but the sign that everything Christ taught is a lie. Such mental inflammation can be caused by a trauma, like the death of a loved one or betrayal by a cleric. It can grow like an unattended cancer. It can set in because we have let ourselves get run down by fast living, a lousy diet (of the sacraments) and no exercise (of faith, hope and charity).

Left untreated, doubt can infect and even kill the soul, just as bodily infections can sometimes kill the body. Treated right, doubt can be turned by the grace of God into an occasion of healing and strength, not only for ourselves, but for others.

Mark Shea is senior content editor at Catholic Exchange. He blogs at NCRegister.com