If it will then we ought to be able to apply this principle to other areas of life. We need to be sincere, but we also need to be right. The patient is sincerely following the doctor's orders, but his orders are wrong. Sincerity will not make the wrong medicine right. It takes more than sincerity to make something true.
Sincerity is certainly a virtue.
There is nothing worse than knowing and believing the truth and being insincere about practicing the truth. However, we must build our lives around truth. John has been warning us about the conflict between light and darkness. Now he is warning us about truth and error, which is another aspect of light and darkness. Those who love darkness love lies and those who love light love the truth.