I know that, for example, eavesdropping a confession is a very grave matter. But I happen not to know anything more about morality of eavesdropping. About a year ago I eavesdropped on my friend’s email communications; I did it because I was afraid of him committing adultery. During the process I thought it wasn’t quite right (my conscience diagnosed it was unkind), but I didn’t know any commandment which would instantly forbid that. Now I am not sure whether this was a grave matter or not (I haven’t used the information I received and he turned out to be loyal to his wife) and whether I have committed a mortal sin. I plan on approaching him on this matter. I won’t be able to speak with my confessor in a meantime, so I’m asking for your help.
The Catechism of the Catholic Church places respect for the truth and for the privacy and reputation of others under the eighth commandment, “You shall not bear false witness against your neighbor.” It could also be argued that unjust interference in someone else’s marriage might fall under the tenth commandment, “You shall not covet anything that is your neighbor’s.”
Before you approach your friend, you may wish to speak with your confessor for advice on whether you should confess to your friend what you did and, if so, what way would be the least damaging way to do so. Be aware that to do so means that you would not only have to confess invading his privacy and email spying, but of suspecting a faithful husband of cheating on his wife.