How Should I Respond to an LGBTQ Person Who Says He Feels Good About Who He Is?

Questions about responding to an LGBTQ person who says he feels good about who he is, and whether—since we all have sin in our life we don’t consider to be sinful—a practicing homosexual can be born again as long as he sincerely believes his behavior isn’t sinful.  

• What would be an appropriate response to an LGBTQ person saying, “I feel good about who I am”?
• Since we all have sin in our life that we don’t consider to be sinful, and will only learn about it when we get to Heaven, doesn’t that mean a practicing homosexual can be born again as long as he sincerely believes his behavior is not sinful?