The Gospel has been preached to all mankind since the Fall. Adam told his children, Seth passed it on. Noah, Japheth, Shem and Ham passed it down to their children. When God scattered mankind and gave them languages many of each language passed the gospel down. So that even the tribes of native Indians had a belief in the Great Father who created the world and would send a savior, many believed that. Were those who accepted the Gospel of John the Baptist and hadn't heard of Jesus any less saved than those who had heard of Jesus. When they heard about Jesus they believed the story because they had believed John.
Sigh.
And look at the world now. People worship cows. People worship trees. People worship statues. The Germans before and during WWII literally worshipped Hitler. The RCC literally worship the Pope/papal office as a whole. The Muslims worship Allah/Mohammed.
Man, innately, knows to worship. But in their fallen innateness, they can not worship God. As Jesus told the woman in John 4, those who worship Him, do so in Spirit and in truth. That is why people worship idols such as trees, cows, statues, fake gods such as Allah, &c.
The gospel has not been passed down to everybody.