They claim to know God, but by their actions they deny him. They are detestable, disobedient and unfit for doing anything good. To say that works are an indication of saving faith is not to make them the basis of one's confidence. It is precisely in this situation that the Law comes into play and convicts the backslidden believer, putting fear in him and driving him to Christ again and again. Backslidden believers do not have assurance precisely because they are not in fellowship with Christ.
Justification is a matter of being declared righteous.
Becoming righteous occurs subsequently, through the process of sanctification. The declaration righteousness is part of justification, but only a part. Not only does God regard us as if we had never sinned. A faulty view of justification causes Hodges et al. The full biblical teaching of justification tells us this is not possible.