God will forgive the worst sinner who truly repents:

(1 Timothy 1:15-16 NIV) Here is a trustworthy saying that deserves full acceptance: Christ Jesus came into the world to save sinners--of whom I am the worst. {16} But for that very reason I was shown mercy so that in me, the worst of sinners, Christ Jesus might display his unlimited patience as an example for those who would believe on him and receive eternal life.

If someone who claims to be a believer persists in their sins it reaches a point where you have to wonder if they're truly saved. A real Christian will never be comfortable living in sin but someone who really isn't saved will be.