The conscience

 Acts 23:1 And Paul, earnestly beholding the council, said, Men and brethren, I have lived in all good conscience before God until this day

God reveals Himself through nature. When you look at it, you do not look at God, but you get to know something about God. Something about His power, deity, and wisdom. It is not entirely enough for man to get saved through such general knowledge of God, but at least it makes Him accountable for His actions. 

Man has an inner witness in him that also testifies about God. And that is his conscience. The conscience is that part of you that helps you differentiate what is right or wrong in any situation you may find yourself in. It is that which points you towards the right direction, which is always in obedience with God's law. 

Everyone alive has a conscience. That is why there are universal laws that govern people and are acceptable everywhere. A slap, a punch in my face, is not acceptable in my country. But even when I do not know all the laws in the new country I relocate to, I am sure it is also not acceptable there. How can that be? Someone placed such a sense in man, that is God. Follow the voice of your conscience; it will keep you out of trouble in many cases. If you meet a person and they tell you about a deal that will get you out of poverty in the next week, you need to be alert before you put your hard-earned money. 

Any sensible person would come to the conclusion that God exists because of the witness of the conscience. For that reason, only fools say that God does not exist. You cannot, therefore, bring an argument before God that you did not know what was right or wrong; He placed a moral compass in you already, hallelujah. 

Further Scriptures

Rom 1:18-19  For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men, who hold the truth in unrighteousness; Because that which may be known of God is manifest in them; for God hath shewed it unto them.

Rom 1:32 Who knowing the judgment of God, that they which commit such things are worthy of death, not only do the same, but have pleasure in them that do them.

Rom 2:14-16 For when the Gentiles, which have not the law, do by nature the things contained in the law, these, having not the law, are a law unto themselves: Which shew the work of the law written in their hearts, their conscience also bearing witness, and their thoughts the mean while accusing or else excusing one another;) In the day when God shall judge the secrets of men by Jesus Christ according to my gospel.