"Whoever believes in him is not condemned, but whoever does not believe stands condemned already because he has not believed in the name of God's one and only Son." - John 3 Verse 18 (NIV)
If you believe in Jesus, you are not condemned. If you do not believe in Him, you are condemned. That's the message in this verse.
People are only condemned if they do not believe in the name of Jesus. "Here is the verdict: Light has come into the world, but men loved darkness instead of light because their deeds were evil" - John 3:19 (NIV).
"Jesus straightened up and asked her [i.e., the woman caught in the very act of adultery], 'Woman, where are they? Has no one condemned you?' 'No one, sir,' she said. 'Then neither do I condemn you,' Jesus declared. 'Go now and leave your life of sin.'" - John 8:10-11 (NIV).
Look at this: "For God did not send his Son into the world to condemn the world, but to save the world through him" - John 3:17 (NIV). Yet in our day it is common for one to stand up before a group of believers and condemn each one of them, the very thing Jesus wouldn't do.
If you are a believer, let nobody condemn you. If you happen to have sinned, Jesus says, "I don't condemn you." But (note carefully) - Jesus expects you to seek His forgiveness, after which you "Go... and leave your life of sin" - John 8:11 (NIV).