In verse 18 we have God addressing a very important question. The fact is that they have heard. Psalm 19:1 states:

Psalm 19:1 The heavens declare the glory of God; and the firmament sheweth his handywork.

Romans 1 already established the fact that all people know of the true God. No one will have an excuse when they stand before the Lord Jesus Christ at the great white throne judgment. In the previous verses in Romans 10, the issue is of the need to remind people of what they inherently know in their conscience already.
Two times in the book of Psalms, God declares that the fool hath said in his heart, there is no God. There are people who claim they do not believe in God. Atheists like to say that, but then why would they even talk about God if they do not believe there is a God?
In our text, Paul was directed to answer the question with a “yes”. The truth has gone out into all the earth. The purposes of evangelists, pastors and teachers is not to surprise people with something they do not know. It is to clarify the facts and to re-emphasize the facts. There are people who plead ignorance, but that is their own fault. They choose not to pursue the curiosity that God has put inside them. The purpose of the faithful servant of God is to take every opportunity to show the lost of the need to deal properly with their guilt of sin.
I have spoken to people who stated that they never heard of the need to be saved when they were younger. I have heard people say that the United church never talks of the need to be saved. Catholics would make the same claim. That is no excuse for not getting saved. Anyone that thinks would know that the Catholics and United, and many other groups are liars. The conscience within the person helps them to know that.
Most people are quite lazy when it comes to spiritual matters. It is interesting to note the time and effort that a person will put into purchasing a car, or investing their savings, and yet when it comes to the spiritual needs, they have no interest. They choose to ignore the most important aspect of life.
I read of a person who recently retired with savings of several hundred thousand dollars. The person had looked at several different options of where to invest his savings to get a little better interest rate. He finally settled on a particular contact he had made. He thought he had done his homework and he invested the entire amount with this contact. After it was too late, he found out they were a fraud. He lost all that money. Then he chose to blame his financial institution with not questioning him enough about why he was withdrawing such large sums of money. It is interesting how people want to be left alone and not questioned, but when things fail, then they want to blame others for not being nosey enough. He is the one who could have asked his financial institution if they thought the contact he was trusting was legitimate. He did not do that.
Back to our text, the fact is that people know. In verse 20 Paul was again directed to quote from the Old Testament. Moses was directed by God to declare that God would provoke Israel to jealousy by a people that Israel did not respect. Gentiles would know the truth and they would proclaim the truth to the Jews. How could that be true. Israel should have been the evangelists to the world, but they rejected that privileged place.
In verse 20 God directed Paul to reference Isaiah on the same matter. Paul stated that Isaiah was very bold in what he stated. Gentiles were trusting in the true God, while Israel was rejecting God more and more.
In verse 21 God declared that He had been faithfully reaching out to them but they were a disobedient and gainsaying people. The word translated as “gainsaying” means to dispute and refuse. That described the Jews of Paul’s day for the most part. It also describes many Jews today. It also describes many Gentiles today.
It is not that people cannot know the truth. It is that people are not interested in the truth. That is the problem. They are too busy and too involved and too preoccupied with other things. They have no time for the Gospel. The day will come when they will wish they had taken the time. As we listen to the media go after the P.C. leader over his weak statements on the sodomite agenda, they are tripping over themselves to try to make the leader out to be a terrible man. The P.M. lied and stated that the statements made by the opposition leader are very dangerous and hurtful to the sodomites. The world has accepted the lie that sodomites cannot help who they are. A reading of Romans 1 would refute that lie. However, to these wicked people Romans 1 is hate speech. They may well throw some people in prison for quoting these passages, but they will be cast into hell one day and then into the lake of fire. I would rather spend a short time in prison, than to spend eternity in the lake of fire.
It is high time that people wake up and realize the foolishness of pandering to wicked ideas. There is a great need for people to believe the truth and be forgiven and saved. God’s salvation provides freedom from the bondage of sin. It provides hope in the midst of a hopelessness that many are facing.
I listened to a influencer yesterday who was interviewing another man. He asked that man if he had any words of hope in the midst of what was going on these days. That man was lost and he had nothing hopeful to offer. Hope is found in knowing the true God and living for Him. God has never failed anyone who has put their trust in Him. His promises are sure. He provides all the blessings that man could ever want. The blessings begin with trusting in God for His salvation. After that momentous occasion, God continues to work in His child to purify that person. That is a rewarding process.

Psalm 100:2 Serve the LORD with gladness: come before his presence with singing.