Gospel of John, Chapter 8 (The Woman Caught in Adultery)
Throughout the Old Testament (and in John 4), we see adultery used as a symbol of man's infidelity to God. God is eternal, and so are his judgments. God is our King, our Law Giver, and our Judge, but before that, he is a lover. This is why he tells us that "I judge no one." (John 8:15) He sees that we are sinful, but to judge us would be to condemn us. In Chapter 8 of John's Gospel, he shows us his infinite forgiveness, but also his authority as our judge.
The Pharisees bring before Jesus a woman caught in adultery, and challenge Christ to fulfill the Mosaic Law by condemning her to death by stoning. If he does, He is a murderer under Roman law, so the Pharisees are forcing Him to choose between Moses and Caesar. Instead, Jesus replies, "Let him among you who is without sin be the first to throw a stone at her." (John 8:7)
This answer defuses the situation, but it also puts things in their proper order. God is the Judge -- not us, nor Moses, nor Caesar. By picking an adulteress, the Pharisees are showing exactly why. If the woman is to be condemned for adultery, then we who are all just as guilty as she is, and deserving of death. But Jesus Christ is offering us a path to forgiveness and eternal life.
Christ first calls us to believe in and follow Him. "I am the light of the world; he who follows me will not walk in darkness, but will have the light of life." (John 8:12) He is saying this to people who don't know whether or not they should believe his testimony. Jesus declares, "I bear witness to myself, and the Father who sent me bears witness to me." (John 8:18) How do the Jews know that the Father has really sent Jesus? They may have seen him work signs. They've heard his teaching. Even so, Jesus is still asking them to put aside their doubts, and to make that leap of faith.
Making that leap isn't enough, though. Once we accept that what Jesus says is true, there is still an obligation to find out what that Truth really is. "If you continue in my word, you are truly my disciples, and you will know the truth, and the truth will set you free." (John 8:31-32) Jesus Christ isn't just a truthful person. He is the Truth. Sin, though, can confuse us and lead us away from God. Because God is also the source of all Life, if we move away from God, we move away from Life and toward death. Ever since the Garden of Eden, death has stalked us, but God is offering us a way out.
"Truly, truly, I say to you, if any one keeps my word, he will never see death." (John 8:51) This is the part of the teaching where Jesus finally wears out his welcome. His audience has accepted that he is telling the truth. They are eager to hear what that truth is. Unfortunately, they're still the same sinful, adulterous people. In an ironic twist, they, the adulterers, pick up stones to throw at Jesus.
Thankfully, that's not the end of the story. God did not make us just to condemn us all to Hell. He truly wants us to be in Heaven with Him. The whole point of sending his Son was to save the world. He commands us to believe in Him, to follow Him, and to obey Him. Through His power and grace, that is possible.