One cannot help be in awe when he contemplates the mysteries of eternity, of life, of the marvelous structure of reality. It is enough if one tries merely to comprehend a little of this mystery every day. Never lose a holy curiosity. Albert Einstein
But the basic reality of God is plain enough. Open your eyes and there it is! By taking a long and thoughtful look at what God has created, people have always been able to see what their eyes as such can't see: eternal power, for instance, and the mystery of his divine being. So nobody has a good excuse.
What happened was this: People knew God perfectly well, but when they didn't treat him like God, refusing to worship him, they trivialized themselves into silliness and confusion so that there was neither sense nor direction left in their lives. They pretended to know it all, but were illiterate regarding life. They traded the glory of God who holds the whole world in his hands for cheap figurines you can buy at any roadside stand.