What does Romans 1:19 mean?
The previous verse began with what should be a frightening statement. Paul said that God is angry at human sinfulness. In fact, God's wrath is revealed against all the ungodliness and unrighteousness of humanity, a terrifying idea since God is all-powerful. Human beings who are not righteous before God do not want to know the truth about Him, so they suppress it, holding it back from themselves and others (Romans 1:18).Now Paul shows that such an attempt to avoid knowing about God is useless. Human beings cannot claim to have missed what God has revealed about Himself in nature. The "them" referenced here are those people who suppress the truth in order to disobey God. This verse says, in no uncertain terms, that God has made Himself plain to those who deny Him, or who disobey Him. Their disobedience is not a result of ignorance; it's caused by rebellion.
The following verses will give more details on what God has revealed to everyone through His creation.