"For since the creation of the world God's invisible qualities -- His eternal power and divine nature -- have been clearly seen being understood what has been made, so that men are without excuse" (Romans 1:20).
Christian's acknowledge two forms of revelation -- special and general. Both are testimonies to the majesty and sovereignty of God. Special revelation involves God's supernatural intervention in the affairs of man. General revelation involves God's witness through creation itself.
Our universe is an example of general revelation; it bears the marks of intelligent design and purpose: "The heavens declare the glory of God; the skies proclaim the work of His hands" (Psalm 19:1). General revelation includes the natural order -- it is a God-ordained testimony that He is Creator and Sustainer: "Yet He has not left Himself without testimony: He has shown kindness by giving you rain from heaven and crops in their season" (Acts 14:17).
General revelation also includes that which God has placed in our hearts; for example moral law: "Indeed, when Gentiles, who do not have the law do by nature things required by the law ... they show that the requirements of the law are written on their hearts, their consciences also bearing witness" (Romans 2:14-15).
God has spoken to us through creation. It is not for lack of evidence that people reject God. General revelation is available to all. Acceptance of this light will lead one to the greater light; rejecting it leads one further into darkness.
"Through Him all things were made; without Him nothing was made that has been made. In Him was life, and that life was the light of men. The light shines in the darkness, but the darkness has not understood it" (John 1:3-5).