In this world there is good vs. evil. Human beings have their good side and their bad side which is evil. If you side with the good, the evil can't overcome you. If you side with evil, the good can't overcome you. It is only through Jesus that we can be forever on the good side. Still, we can side with the good and allow evil to overcome us at times. However, that doesn't mean that we aren't still on the good side. From the moment that we give our lives to God through His Son Jesus then He will never let us go. So no matter how horrible we think we are, no matter how bad we've been, He will never let us go.
"Even if we are unfaithful, He remains faithful because He cannot be untrue to Himself." (2 Timothy 2:13)
"For it is by grace that you've been saved through faith, and that not of ourselves. It's the gift of God, not of works, so that no one can boast." (Ephesians 2:8-9)
"I will never leave you nor abandon you." (Joshua 1:5, Hebrews 13:5)