Here two universal things that give us knowledge of God are mentioned here in Romans 1:18-20, yet men suppress the truth:

1. God has made knowledge of Himself evident within us.  We just know that He is. To be an atheist is to suppress this knowledge. Romans 1:18
2. His power and divine nature are seen in creation. Romans 1:19

Still people suppress this truth about God. Attempting to put God out of our minds just brings trouble.

1. People hold or suppress the truth in unrighteousness. Romans 1:18
2. People do not glorify God as God and are unthankful. This leads to darkness. Romans 1:21
3. When not acknowledging God any longer God gives people over to depravity. Romans 1:28

All people know instinctively that they do not measure up as they should.

1. The hypocrite that judges another will be judged for the same things. Romans 2:1-3
2. Those sinning without the law will perish without the law. Romans 2:12
3. Those sinning under the Law will be judged by the Law. Romans 2:12
4. Those without the law still sin and their consciences show that they have the work of the Law written in their hearts. This is shown when their thoughts either accuse themselves or making excuses for  themselves.  Romans 2:14-15

The conclusion here is that we are a sinful lot quite incapable of keeping the law. Forget any notion that you can keep the ten commandments and thus satisfy God. None of us can do that, not all of it, not all the time.  To be guilty under God's law is an incredibly bad thing. It is a good thing that He has provided for our salvation through faith in the Lord Jesus. 

John 3:36  New American Standard Bible (NASB)

36 He who believes in the Son has eternal life; but he who does not [a]obey the Son will not see life, but the wrath of God abides on him.”

Footnotes:

a John 3:36 Or believe

Romans 3:19-20   King James Version (KJV)

19 Now we know that what things soever the law saith, it saith to them who are under the law: that every mouth may be stopped, and all the world may become guilty before God.
20 Therefore by the deeds of the law there shall no flesh be justified in his sight: for by the law is the knowledge of sin.

In another place the scriptures points out that the Law came after the promise made to Abraham and that the Law could not and cannot give life. Clearly we need something else.

Galatians 3:21-24   King James Version (KJV)

21 Is the law then against the promises of God? God forbid: for if there had been a law given which could have given life, verily righteousness should have been by the law.

22 But the scripture hath concluded all under sin, that the promise by faith of Jesus Christ might be given to them that believe.

23 But before faith came, we were kept under the law, shut up unto the faith which should afterwards be revealed.

24 Wherefore the law was our schoolmaster to bring us unto Christ, that we might be justified by faith.

At this point I want to speak a bit about what righteousness and unrighteousness are.

Righteous means to be free from wrong, guilt or sin. The chart below will show some synonyms of righteousness in one column and the opposite in the other column.  

Justification is the act of showing or proving to be just or conformable to the law. When God justifies us He changes our status before Him. Much as in a modern US court where the declaration of the court determines our future so also does God's action on our behalf to justify us changes our status before Him with immeasurable blessings for us. 

Synonyms and Opposites

We have shown above that the law does not justify us, but it does show us our sinfulness and it condemns us. Now we will begin to talk about the solution.

1. The law and the prophets spoke of the righteousness of God without the law. Romans 3:21
2. This is the righteousness of God for those who trust Jesus Christ. This righteousness is for all who believe. Romans 3:22
3. Now just in case you think you are different, that you are better than most people and you are OK so that you do not need this Paul tells us that there is no difference, there is no distinction. Every last one of us has sinned and fallen short of the glory of God.  Every last one of us needs the righteousness of Jesus Christ by putting our faith or trust in Him.  Romans 3:22-23
4. We are justified freely by His grace by the redemption that is in Christ Jesus. This is truly a free gift from God Himself. Romans 3:24

You might be thinking if God is just how can He do this? We would not think well of one of our judges letting evil doers go without punishment after they had been convicted of a crime.  God took care of this problem in a very public way when He set forth His Son, Christ Jesus to die on the cross for our sins.

1. God set forth Christ to be a propitiation for us. This word is not one that we use often in everyday language but is is very important. It speaks here of the work of Christ to satisfy the anger of God for unrighteousness.  Romans 3:25
2. He did this by shedding His own blood on our behalf. Romans 3:25
3. By this public display He showed His righteousness. He is just and the justifier of the one, anyone, who believes in Jesus. Romans 3:26

Romans 3:27-28 King James Version (KJV)

27 Where is boasting then? It is excluded. By what law? of works? Nay: but by the law of faith 28 Therefore we conclude that a man is justified by faith without the deeds of the law. 

Have you trusted Christ Jesus to be your savior?  If you have not I strongly encourage to do so. Failure to do so has dire consequences.