A man may be very sincere in believing a lie, and he will be so much the more sincere as his heart is the more hard. If his heart is very hard, he will lay aside all candor and will settle down in the belief of a lie so firmly that no evidence of any truth, however palpable, will in the least, move him from his falsehood. It will not be impossible for him to believe any lie, however palpable; and he will not only believe it, but he will give himself entirely up to its control; and the harder his heart is, the more confidence will he have in it, and the fewer misgivings as to its truth.
When a person believes a lie, and gives himself up to its influence, however sincere may be his belief, yet he is without excuse; for he creates his delusion by his own voluntary wickedness — it is forced upon him by no one.
It is only when the judgments or opinions are formed in a right state of heart, that a person is justified in acting in conformity to them. Many people seem to suppose that a person is pursuing a virtuous course when he acts in conformity to his real opinions, whether they are right or wrong, provided he is only sincere. Now sincerity itself may often be an evidence of great wickedness. For a man could not be sincere in pursuing a wicked course of life, or in holding on to a wrong sentiment, if his heart was right.
Therefore, a man is without excuse, who does wrong, however sincere he may be in the wrong he is doing.
Finney Sermon Collection, Volume I, Hardness of Heart