This question, typically voiced with indignation, is the response of a legalist to discovering there is the possibility—or even just the suggestion—that they might be unknowingly sinning.
A healthy believer would respond: “If that is biblical I want to know more! How did you come to that conclusion? Can we open the scriptures together?”
Only someone whose sense of comfort in regard to their relationship with God derives from feeling they are performing well, responds with indignation to the idea they might be unknowingly sinning.
A healthy, mature believer is in touch with the reality that they are utterly dependent upon God’s mercy and grace. And, consequently, is motivated to improve their reflection of Christ’s character not out of any hope of deserving, but out of gratitude and passionate allegiance to Jesus as their Savior and King.
Therefore new realization is an opportunity, not an indictment.