'Sincere' in the Bible

"The eye is the lamp of the body. Therefore if your eye is sincere, your whole body will be full of light.

Your eye is the lamp of the body. When your eye is sincere, your whole body is full of light also. But when it is evil, your body [is] dark also.

so that you may approve what is superior, in order that you may be sincere and blameless in the day of Christ,

{remembering the} sincere faith in you, which lived first in your grandmother Lois and your mother Eunice, and I am convinced that [is] in you also,

Having purified your souls by your obedience to the truth for sincere brotherly love, love one another fervently from the heart,

Dear friends, this [is] already the second letter I am writing to you, in [both of] which I am attempting to stir up your sincere mind by a reminder,