Conscience is God’s spy and man’s overseer, Domesticus index, iudex, carnifex [household chaplain, judge, executioner]; God’s deputy judge, holding court in the whole soul, bearing witness of all a man’s doings and desires, and accordingly excusing or accusing, absolving or condemning, comforting or tormenting. Quid tibi prodest non habere conscium, habenti conscientiam? [what profit is there in not being conscious of having a conscience?] saith one [Seneca]; and another[Ansonius], Turpe quid acturus, te sine teste time. Inprimis reverere te ipsum [when about to act evilly, respect yourself, though there is no witness]. Look to conscience.
"Conscia mens ut cuique sua est, ita concipit intra,
Pectora pro facto spemque metumque suo." (Ovid) [according to the state of a man's conscience, so in his mind do hope and fear arise on account of his deeds]
Commentary on 1 John 3:20