Imagine you had a pile of speeding fines against you. The judge said you either had to pay thousands of dollars or go to jail. You couldn't say you were sorry, (though you should be), it wouldn't get you out of your debt. You couldn't somehow reference a good deed done in the past to try and reconcile these speeding fines. You also couldn't deny they were yours....because well, the evidence is overwhelming against you. There was nothing you could do. You didn't have thousands of dollars. Helpless and hopeless you start imagining your life in jail. But then..... someone says to the judge, "your honor, I will pay his fine!" As your rescuer pays the court clerk for your many fines, the judge accepts your pardon. The person who stepped in to pay your debt met the demands of justice. The scales of justice were upheld, and though you were guilty, you were legally pardoned! "Amazing," you thought. Your gratitude towards this one who paid what you owed was immense. You set your mind on never receiving another speeding fine.
This picture is what it's like to be pardoned in Christ. You are a sinner. A criminal in God's courtroom with the evidence piled high. The One who made your mind can see right into it and knows all the hidden sin and darkness. He has recorded all the sinful deeds, words, and thoughts. The Law and its demands stands as your accuser. Lied-guilty. Stole-guilty. Blaspheming the name of God (even omg)-guilty. Lusted (adultery at heart)- guilty. (The ten commandments are God's law) And so on and so on.
Many don't want to view God like this. They want to think that He is all loving and all forgiving. While He is love (I John 4:8), He is also a Judge. Psalms 97:2 says righteousness and justice are the foundation of His throne. As a Judge (a perfect Judge) He must uphold justice. To expect Him to turn a blind eye to sin is to make Him less than an earthly judge, who (if he's a good judge) must uphold the standard of justice. God's standard, however, demands perfection in thought, word, and deed. Since He is a perfect Judge He must uphold perfect justice.
So here's the grim picture: You stand before God guilty. Saying I'm sorry (though you should be) will do nothing to pardon you. Trying to appeal to some former good deed in your life will not undo, or absolve, the guilty verdict. You couldn't deny the accusations, because, well, He's God and sees everything. The realization that hell is your destiny. Helpless and hopeless there's nothing you could do to get yourself out of this situation. You simply throw yourself at the mercy of the Judge. BUT THEN.... Jesus steps in and says, "I will take his punishment." Because Jesus is perfect His life is the perfect sacrifice. He is the Lamb of God, who takes away the sins of the world. The Bible says without the shedding of blood there can be no forgiveness of sin (Hebrews 9:22). In our analogy, the cost for the speeding fines to be removed was monetary, but the cost for sin to be removed was the blood of an innocent. The realization that you are guilty of breaking God's law, but that Jesus stepped in and paid your eternal fine with His life's blood overwhelms your soul. This is the best news you've ever known. Your response is to stay away from that thing that put your Savior on the cross-sin (as best you can) and to follow Him.
Charles Spurgeon once said this about God's justice and love.
"Learn ye, my friends, to look upon God as being as severe in His justice as if He were not loving, and yet as loving as if He were not severe. His love does not diminish His justice, nor does His justice, in the least degree, make warfare upon His love. The two things are sweetly linked together in the atonement of Christ. But, mark, we can never understand the fulness of the atonement till we have first grasped the Scriptural truth of God's immense justice. There was never an ill word spoken, nor an ill thought conceived, nor an evil deed done, for which God will not have punishment from some one or another. He will either have satisfaction from you, or else from Christ. If you have no atonement to bring through Christ, you must for ever lie paying the debt which you never can pay, in eternal misery; for as surely as God is God, He will sooner lose His Godhead than suffer one sin to go unpunished, or one particle of rebellion unrevenged."