How does he know?

Senator David Vitter, caught in the DC madam brouhaha, said, according to the Washington Post, “This was a very serious sin in my past for which I am, of course, completely responsible. Several years ago, I asked for and received forgiveness from God and my wife.” Now, I’m not a believer, but how does one know that one has received forgiveness from the almighty? Isn’t there a certain lack of humility in such a statement?