Conscience is something, in a sense, apart from man. It has been put in him by God; it is a reminder of the voice of God within him, an inward monitor, and a man cannot really manipulate his conscience. He can go against it, but that is not manipulating it. It is possible, as this Apostle says again in writing to Timothy, for the conscience to be seared “with a hot iron”. But nevertheless it is true to say that the conscience is an independent witness.
Martyn Lloyd-Jones (God’s Sovereign Purpose)