Sin is to a nature what blindness is to an eye. The blindness of an evil or defect which is a witness to the fact that the eye was created to see the light and, hence, the very lack of sight is the proof that the eye was meant... to be the one particularly capable of seeing the light. Were it not for this capacity, there would be no reason to think of blindness as a misforture.
Often a non-Christian knows something about the earth, the heavens, and the other parts of the world, about the motions and orbits of the stars and even their sizes and distances... and this knowledge he holds with certainty from reason and experience. It is thus offensive and disgraceful for an unbeliever to hear a Christian talk nonsense about such things, claiming that what he is saying is based in Scripture. We should do all that we can to avoid such an embarrassing situation, which people see as ignorance in the Christian and laugh to scorn.
If you would attain to what you are not yet, you must always be displeased by what you are. For where you are pleased with yourself there you have remained. Keep adding, keep walking, keep advancing.