"Even when Gentiles, who do not have God's written law, instinctively follow what the law says, they show that in their hearts they know right from wrong. They demonstrate that God's law is written within them, for their own consciences either accuse them or tell them they are doing what is right."  Romans 2:14-15

Conscience is not an instinct, it is a knowledge, a consciousness put within us by God, it is one of the things that differentiates us from animals. Though the conscience cannot act, it can and does judge. It takes our thoughts, words, deeds and motives and compares them with the moral law of God, with the will of God. Then it passes its judgment.

The Bible is teaching that everyone, even if they've never heard about God's Word, knows they should act according to God's morality by nature. The 18th-century German Philosopher Immanuel Kant said, "Two things fill the mind with ever increasing wonder and awe, the more often and the more intensely the mind of thought is drawn to them--the starry heavens above me and the moral law within me." (Immanuel Kant, Critique of Pure Reason, 1781).

There is a "law" or a "morality" operating in everyone's own heart either accusing us or telling us we are doing what is right. Because of this, we are responsible to do what is right and to avoid what is wrong or sinful, and when we violate our conscience, we feel guilt. 

Psalm 36, verses 10-12, "Pour out your unfailing love on those who love you; give justice to those with honest hearts. Don’t let the proud trample me or the wicked push me around. Look! Those who do evil have fallen! They are thrown down, never to rise again."

Proverbs 6, verses 6-8, "Take a lesson from the ants, you lazybones. Learn from their ways and become wise! Though they have no prince or governor or ruler to make them work, they labor hard all summer, gathering food for the winter."

Stay strong in the Lord,