Open discussion part 2: What is sin?

How can we truly judge what is sinful when we are not concrete on what sin is? There are basic commandments that everyone agrees is wrong but then we are left we a lot of grey areas.  Paul said everybody needs to work out their own soul salvation. Good idea, but then people used that to mean if God don't convict me then I must not be wrong. However, we failed to be honest that we have not been consistent in our prayer life for God to convict and show us ourselves. How can we know what God wants personally from us when we don't talk with Him?