When God created man He set a conscience within his soul so that he may be governed by it as by a rule, and so that he may be guided in what to do and what to avoid. Conscience is nothing other than natural or innate law, which is why it also agrees with the written Law of God. For whatever the Law of God teaches, conscience teaches also.
The Law of God commands us to know the one God; conscience teaches the same. Wherefore even pagans, convinced by conscience, acknowledged one God.The Law of God commands us to reverence God above all creatures and to render supreme honor to God alone; conscience teaches the same. The Law of God teaches us not to hold anything equal to God; conscience teaches the same. The Law of God commands us to show submissiveness, subordination and obedience to God as the supreme authority; conscience teaches the same. The Law of God commands us to fear God as the most righteous Judge; we hear the same from conscience. The Law of God commands us to punish those that blaspheme the name of God; our conscience cannot endure it either. The Law of God teaches us to listen to God more than to man; conscience teaches the same. The Law of God demands that we love God above all things; conscience demands the same. For God is the most exalted and supreme good and the source of all good, therefore we must love Him above all created good things.The Law of God teaches us to thank God for all things; conscience teaches us the same, for conscience itself convinces us to be thankful to our Benefactor.The Law of God commands us to put all our hope in God; conscience commands the same. For God alone is able to do all things as Almighty. Every created thing is powerless without God and apart from God, therefore hope in them is inconstant and infirm. The Law of God commands us to submit and give honor to parents and all authorities sent by God; we hear the same thing from our conscience.The Law of God forbids us to harm a man and to deprive him of life; conscience forbids the same thing. The Law of God commands us to help a man in misfortune and need; conscience commands the same. The Law of God forbids adultery and fornication; a man hears the same from his conscience, and it thunders within him not to defile himself with uncleanliness.The Law of God forbids us to lay hold of the goods of others without the consent of the proprietor; conscience cries out the same. The Law of God forbids lying, flattery, and deceit; conscience forbids it too. The Law of God commands us not to desire anything that belongs to another; conscience commands it too. Thus the Law of God and conscience mutually agree and are appointed for the same end, that is our blessedness, whence even the pagans, enlightened by philosophical teaching wrote many useful precepts. This comes from nothing other than conscience or natural law illumined by much labor and instruction.Therefore everyone who sins against conscience also sins against the Law of God and against the Lawgiver Himself, God. Whoever does not listen to his conscience listens neither to the Law of God nor to God Himself. Whoever does not obey his conscience obeys neither the Law of God nor God Himself. Whoever offends his own conscience also offends God Himself.Those Christians who sin against their conscience do not truly worship God, but are hypocrites, for it is impossible to worship God without a clean conscience. A true Christian does not wish to, and keeps himself from sinning against his conscience, and so break the Law of God. He would rather suffer than sin. Faith rests in such a conscience and makes a man joyful; for where there is a clean conscience, there is faith and joy. Just as the Law of God accuses a man of sin, so likewise conscience accuses him also. So it is that when a word of reproof is spoken in general, then sinners are wounded in conscience and troubled.Thus the profligate are troubled when profligacy is spoken of; thieves and robbers are troubled when thievery and robbery are spoken of; flatterers and liars are troubled when deceit and lies are spoken of; they are troubled and even show some outward signs. This is the accusation of conscience working in them.Just as the Law of God puts the fear of God's judgement into a sinner, so likewise conscience puts fear into him and cries within the sinner, "Man, it shall go miserably for you." Just as the effect of the Law of God and of conscience are the same, so shall they be the same at the Judgement of Christ. There the Law of God which he violated will accuse the sinner; the conscience offended by his sins will also accuse him.There, these two, conscience and the Law of God, will be the witnesses and the accusers against every sinner. It happens that an evil conscience is as though asleep; but when it awakens and begins to accuse the sinner, then cruel torment will come upon him through his conscience, whence it is that many kill themselves, not enduring the pangs of conscience.For just as there is no better repose than from a pure conscience, so likewise there is no greater disquiet and torment than from a wicked conscience. If conscience torments so much here, how shall it torment a sinner in the age to come when all his sins shall stand before him and it accuses him of them and torments him?O sinners, why do we sleep? Let us awaken and repent and cleanse our sins by repentance and contrition of heart, and let us correct ourselves and cease from sinning and offending our conscience, lest we appear before the Judgement of Christ with an evil conscience blackened with sins, when the books of the conscience shall be opened and each shall receive according to his works.Do not do what conscience forbids you to do, for an unerring conscience forbids what the Law of God also forbids. For a good conscience is in agreement with the Law of God. The Law of God says, "Thou shalt not kill, thou shalt not steal" (Ex. 20:13,15), and so on. You also hear the same within your conscience, and it tells you the same thing. Avoid, then, doing what conscience forbids, lest having wounded your conscience you wound your soul.
from: Journey to Heaven Counsels On the Particular Duties of Every Christian Our Father Among the Saints, Tikhon of Zadonsk, Bishop of Voronezh and Elets Jordanville, NY: Holy Trinity Monastery, 2004.