A Pebble Thrown into a Pond

Have mercy on me, O God, according to your steadfast love; according to your abundant mercy blot out my transgressions.
Wash me thoroughly from my iniquity, and cleanse me from my sin.
For I know my transgressions, and my sin is ever before me.
Against you, you alone, have I sinned, and done what is evil in your sight, so that you are justified in your sentence and blameless when you pass judgement.
Indeed, I was born guilty, a sinner when my mother conceived me.
—Psalm 51:1-3

As well known as Psalm 51 is, it's probably not prayed as often as it should be. In this era when we have decided that there is no such thing as sin, this psalm seems to offer nothing but unnecessary guilt. Why should I ask a loving God for mercy? How can it be that I have sinned only against God? How dare anyone imply that there's such a thing as original sin?

And yet here in the first five verses of Psalm 51, these are the things that I pray. So I'll answer the aforementioned questions in reverse.
  1. I am opposed to war. I am opposed to breaking up families. I am for education. I am for healthcare. And yet I participate in—indeed I support with my finances—a system that makes weapons of mass destruction, cages children apart from their mothers and fathers, defunds public schools, and spends next to nothing to provide healthcare to those who need it most. This is an example of original sin. I say I'm against these things, but am I to be held blameless when I benefit from the very system I disagree with? This happens to all of us throughout our lives. We consider ourselves blameless in a particular area, and yet the world's sin invades our lives, sometimes ruining them. Secondhand smoke, drunk driving, crime—I do not do these things, and yet I am vulnerable to their effects. And there are practices that I participate in which to me seem perfectly innocent, yet which profoundly impact others: the food that I eat, the clothes that I wear, the products that I use—many of these things are made under conditions that hurt people, yet I say I have no control over what I do. Indeed, I was born guilty...
  2. The Bible stresses that humankind was made in God's image. And yet not all people care to imitate the Divine in their dealings. Indeed, some are quite easily judged as being undeserving of our Christian consideration. Yet Calvin said that "we are not to look to what [people] in themselves deserve, but to attend to the image of God, which exists in all, and to which we owe all honor and love" [Institutes 3.7.6]. When I hurt another, even if I think they've received their just desserts, I hurt the image of God in which they were created. And so it's true that when I commit wrongdoing, it is ultimately God who has been sinned against.
  3. And finally, why should I beg God for mercy if God loves me and forgives me anyway? Besides, what I do doesn't matter in the scheme of things. And yet, everything I do, every action I take is like a pebble thrown into a pond. Its ripples radiate outward and have an effect on everything around me. To practice confession is to acknowledge that what I do matters, and to invite God into my desire to change, to improve. Contrition isn't about a pointless rule, but about acknowledging my need for a relationship with a Creator to whom I've caused pain. 
Have mercy on me, O God, according to your steadfast love; according to your abundant mercy blot out my transgressions. Wash me thoroughly from my iniquity, and cleanse me from my sin. For I know my transgressions, and my sin is ever before me. Against you, you alone, have I sinned, and done what is evil in your sight, so that you are justified in your sentence and blameless when you pass judgement.  Indeed, I was born guilty... Amen.
—Psalm 51:1-3