In her 1922 edition of Etiquette, Emily Post mentions over one hundred occasions and ways in which a person should thank another. For Post, whose book served as the standard for social interaction for generations, the idea of failing to express gratitude constituted a major breach in manners. Most would agree. But for Catholics, a deeper question remains: could ingratitude constitute a sin?

Saint Thomas Aquinas addresses this question in the Summa Theologiae with a powerful condemnation of ingratitude. We might expect to find his question under the heading of charity, yet Aquinas considers gratitude a component of the cardinal virtue of justice; thus, ingratitude violates a cardinal virtue. 

As such, Aquinas explains, “every ingratitude is a sin.” 

While Aquinas argues that most sins of ingratitude are venial, he writes that some can be mortal. He explains that “a man may be ungrateful, because he not only omits to pay the debt of gratitude, but does the contrary. This again is sometimes mortal and sometimes a venial sin, according to the kind of thing that is done.” 

In his treatment of gratitude and ingratitude, Aquinas relies extensively on the Roman orator Seneca’s On Benefits. For his part, Seneca was harsher on ingratitude than Aquinas. Seneca writes, “while the greatest vices are common, none is more common than ingratitude…” Seneca writes:

Among the numerous faults of those who pass their lives recklessly and without due reflexion…I should say that there is hardly any one so hurtful to society as this, that we neither know how to bestow or how to receive a benefit.

It is easy to see how ingratitude—the lack of thankfulness and appreciation for good works, gifts and kindnesses—devastates marriages and families, but Seneca posits that it hurts entire societies. 

Ingratitude can also demolish our relationship with God. Consider the miracle recounted in the Gospel of Luke in which Jesus cured ten lepers, yet nine did not bother to thank Him. I might be baffled by their responses, but honesty compels me to wonder how many times I have acted just like the nine. I wonder how many times I have considered something more important than thanking Jesus—or worse, been discontented in the things God has given me. After almost half a century on this earth, I have come to understand that the sin of ingratitude leads to other sins. 

But going forward, I have a hero and a template for the special virtue of gratitude: the tenth leper. As the Gospel tells us: “Then one of them, when he saw that he was healed, turned back, praising God with a loud voice; and he fell on his face at Jesus’ feet, giving him thanks. Now he was a Samaritan.” I want this to be my response: to fall at the feet of God and glorify Him with a loud voice. To recognize that while I deserve nothing, God has given me everything. He has forgiven my sins. He has given me Himself. It’s a thought we all must ponder. 

We must also recognize that true gratitude toward God naturally extends to thanking others. Perhaps we should begin with those people we forgot to thank in the past. While Aquinas argues that every benefactor must be thanked, justice does not demand that they are thanked immediately. Thus, we need to think about those persons in our lives whom we have never properly thanked. Maybe it’s a college professor who taught you many years ago. Maybe it’s your own mother who raised you. Maybe it’s someone who offered a kind word that changed your life for the better. 

You might be thinking it’s too late—that your benefactor has long forgotten his kind deed. And maybe he did forget. Show him you remember. As Seneca put it, “It is ungrateful to take no notice of a kindness, it is ungrateful not to repay one, but it is the height of ingratitude to forget it.”

Ultimately, it’s not only polite to follow Emily Post’s advice to properly express gratitude toward others; it is a moral duty and an essential virtue. 

So, who can you thank today?