The truth of his existence.
(check on Rom 1:18-21)
God made man and he hates sin.
Man knows this. Even his conscience bears witness on these matters.
The invisible things, his eternal power, his grace and love, things like that, abound in everyone's everyday lives. otherwise, everyone'd be dead. (i.e. God does know all these sins, and hates the sins, but loves the people and so spares them)
So people DO know that God exists. People find themselves turning to the supernatural (e.g. some other thing that is more than man, much more powerful. you can quote superman or psychics, or aliens or kung fu, mutants or jedi, but the point is, people do long for the supernatural, and thus bear witness, through their longing for Him without knowing exactly what it is they're longing for)
In other words, people KNOW, in their hearts and spirits, if not in their minds, that God is real.
But although they know him in that sense, they refuse to acknowledge him, making themselves as gods and worshipping their own intellect and prowess, claiming that He does not exist and making elaborate theories to explain what has been His work (Romans 1:21,22), turning to idolatry and worshipping things made by their own hands (verse 23)
so comes his wrath. (verses 24,25)