I responded, "looking at pornography on the internet". While that particular sin is definitely quite harmful and very serious, ten more years of experience has led me to change my mind, sins of the tongue are probably the most insidious because they are both very common and extremely damaging to all involved in them. The Apostle James tells us in his epistle that
"no human being can tame the tongue. It is a restless evil, full of deadly poison. With it we bless our Lord and Father, and with it we curse people who are made in the likeness of God" (James 3:8-9).
Let us be honest with ourselves, we can take sinful pleasure in speaking badly of people not in the room. Some of us can even enjoy insulting people to their face
(the sin of reviling). But most of us lack the courage to do that often, and thus limit ourselves to speaking negatively about someone behind their back. Sinning in this way can take the form of revealing a fault or shortcoming of someone to another who has no need to know it
(the sin of detraction). Or one can deliberately tell a falsehood about someone's bad behavior
(the sin of tale bearing), i.e., saying "David is having sex with his girlfriend" when one knows very well that David is in fact respecting the sanctity of the marriage bed and being chaste. One can also make fun of someone in a mean-spirited way, either to one's face or behind one's back
(the sin of derision). People have a right to a good reputation. When we speak poorly of them, especially when we are less than entirely honest, we rob them of this right.
Our Lord is very clear about our obligations to others when it comes to speech "Say good things (viz., bless) about those who curse you". Indeed Jesus warned us that "every idle word that men shall speak, they shall give account thereof in the day of judgment". (Mt. 12:36).
As we prepare ourselves to go to confession at the penance service at Saint Paul on September 11, we do well to take stock of our sins of the tongue in order to tell God we are sorry and to ask for His help to avoid these sins and their near occassion in the future, with the help of His grace.