Did Jesus die for Neanderthals and how could they hear about the gospel message? Not joking, I'm seriously curious from the Christian perspective.
That's us.
All modern humans have Neanderthal DNA, new research finds
But to answer your question more directly:
Romans 1:20
For his invisible attributes, namely, his eternal power and divine nature, have been clearly perceived,
ever since the creation of the world, in the things that have been made. So they are without excuse.
Romans 1:19
For what can be known about God is plain to them, because God has shown it to them.
Isaiah 40:26
Lift up your eyes on high and see: who created these? He who brings out their host by number, calling them all by name, by the greatness of his might, and because he is strong in power not one is missing.
Psalm 19:1-6
To the choirmaster. A Psalm of David. The heavens declare the glory of God, and the sky above proclaims his handiwork. Day to day pours out speech, and night to night reveals knowledge. There is no speech, nor are there words, whose voice is not heard. Their voice goes out through all the earth, and their words to the end of the world. In them he has set a tent for the sun, which comes out like a bridegroom leaving his chamber, and, like a strong man, runs its course with joy.
Psalm 33:6-9
By the word of the Lord the heavens were made, and by the breath of his mouth all their host. He gathers the waters of the sea as a heap; he puts the deeps in storehouses. Let all the earth fear the Lord; let all the inhabitants of the world stand in awe of him! For he spoke, and it came to be; he commanded, and it stood firm.
Psalm 8:3
When I look at your heavens, the work of your fingers, the moon and the stars, which you have set in place,
Romans 2:15
They show that the work of the law is written on their hearts, while their conscience also bears witness, and their conflicting thoughts accuse or even excuse them
Deuteronomy 4:19
And beware lest you raise your eyes to heaven, and when you see the sun and the moon and the stars, all the host of heaven, you be drawn away and bow down to them and serve them, things that the Lord your God has allotted to all the peoples under the whole heaven.