1 Peter 4:6 For this cause was the gospel preached unto them that are dead, that they might be judged according to men in the flesh, but live according to God in the spirit.
One of the mysterious Biblical truths that we must believe because we cannot fully understand it is that we are at the same time and all the time saint and sinner. The promise is that we are saints, holy people inside who never sin but only do good. We don’t have so much trouble believing we are sinners, for, unless we are prideful Pharisees, we can plainly see our selfish sinfulness. But the saint, the holy person inside, is not so readily apparent. Thus, God grants faith to believe it: “I am righteous, I am holy, I am a saint.” The saint lives according to God in the spirit.
That is the promise that is not often seen but yet is believed. Faith sees that I am a saint who only does good (for which God gets the credit), and I am a sinner who can only do sins (for which I am justly judged). When I do something bad I get the blame; when I do something good Jesus in me gets the credit. I can take no credit for any good work I have done.
The gospel preached to us who are dead in sin; the sinner is judged. If I believe the Gospel then my sin is judged in the cross of Christ. The sinner “dies daily” and the saint lives.