You don't need to share the gospel in order to convince someone that people can be really bad. All we have to do is live in a family, work somewhere, go to school, look in a mirror, or do anything that involves relating to people on a daily basis and we can see that people can be really bad.
You don't need to share the gospel in order to explain that there are competing world views out there. Understanding this does not need a redeemed mind; an unredeemed mind can understand this.
You don't need to share the gospel in order to prove to others that some people live empty, sad lives, and that people are looking for meaning in their lives. Those who have never heard the gospel can see for themselves (and maybe even experience themselves) these realities.
You do need the gospel if you want to give people a remedy for these realities. All of these things, the sad condition of the world, the emptiness of people, the proliferation of competing world views apart from God, are all symptoms of something that the gospel deals directly with: sin. Identifying these ills is not the issue; that's easy. An unregenerate person can identify them. But does simply knowing them give a solution? Is this a case of "knowledge is power?" I would very humbly say no. The gospel gives us the resolution. The gospel emphasizes three very crucial things:
- God is holy, and we are not.
- We are sinners, cannot approach God on our own merits, and without Him we will suffer eternal separation from Him.
- Trusting in the sacrifice of Jesus Christ provides us with a way to approach God and be spared from this eternal separation.
So faith comes from hearing, and hearing through the word of Christ. (Romans 10:17)