It doesn't take one to be an atheist to buy the lie that man is basically good. Some practice some form of liberation theology where they believe that man is basically good because God made man. It's true God made man but ever since the fall one must realize that man is not basically good. Just tell them they're sinners and they're ready to tear people apart. The problem is that this heart that's so hardened in sin may love sin a lot while thinking nothing is wrong with one's spiritual welfare. Many live so sinfully while declaring themselves righteous. It's because people tend to think that just because they're religious then they're good people. James 2:10-11 says you need to keep the whole Law if you want to work for your salvation.
The great preacher C.H. Spurgeon said, "The proof of man's being unsaved is not only evident in self-righteousness but a wicked lifestyle. What is disturbing is that many people think they are "good" even if they love to live a life of sin. Many religious but lost people are faithful to their religion but are also degenerate in their lifestyles at the same time. When people are truly saved, they are delivered from their sinful lifestyles by the grace of God so nobody can boast." The proof of being unsaved can even go hand in hand with both a wicked lifestyle and self-righteousness. In short, works salvation ends up as a license to sin. Because many think they can live in habitual sin as long as they do enough good works to make it.
Proverbs 14:12 warns that the ways of man seem right to them but it leads to death. How many sins today increase all because people think that they're basically good. Instead of addressing the fact man is a sinner, many times we really end up thinking that they're really good people never mind that people do wrong because they're sinners. Yet so many of them still think they're good enough for Heaven because they're faithful to their religion or they do more good than bad. Many people today are religious, live sinfully and are very well loved by the world. On the other hand, let a Christian fall down like David did and most of the world will still continue to scorn that person even if the person has already repentance.
Do you want the way of life? Proverbs 6:23 calls the reproofs of instruction as the way of life. The Law of God is the very way of life. The problem is have we really kept it? Galatians 3:24 shows that the very Law of God is what brings us to the knowledge that only Jesus saves. Without the Law of God, men are left ignorant about their sin. Yet many people hate His Law as Psalm 119:10 that salvation is far from the wicked because they hate God's precepts. What's important is that the whole Gospel must be preached because 10 true converts means a lot more than having 100,000 false converts just to fill the churches.