The night of the Last Supper our blessed Lord told of the three effects of the Holy Spirit. One of them was: “He will convict you of sin, because you did not believe in me.”
How do we know we are sinners? By the Holy Spirit.
Not because we break the law. There isn’t a driver who hasn’t broken the law against speeding. Did you ever lean over your steering wheel and say an act of contrition? Nobody is really sorry for breaking a law, unless he gets caught. The law is for the imperfect. Our blessed Lord related sin to unbelief – “Because you did not believe in me.” If we did not believe in him, we crucified him.
So what is sin? Sin is hurting someone you love. That is sin. That is why the crucifix reminds us of our sin.
The life of each and every one of us has been written. The crucifix is my autobiography. The blood is the ink. The nails the pen. The skin the parchment. On every line of that body I can trace my life. In the crown of thorns I can read my pride. In the hands that are dug with nails, I can read avarice and greed. In the flesh hanging from him like purple rags, I can read my lust. In feet that are fettered, I can find the times that I ran away and would not let him follow. Any sin that you can think of is written there. This is what he Spirit does for us.