God is the self-revealing God.
Paul underscores that he is clearly revealed to every person through creation:
For what can be known about God is plain to them, because God has shown it to them. 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 1:19–20 ESV
And also through conscience:
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.
Romans 2:15 ESV
This knowledge of God is so clear to the whole of mankind that they are without excuse. By virtue of God’s arrangement with our first parents the whole of humanity, regardless of belief, are in covenant relationship with God. Some, through faith in Christ as the last Adam and the second man, are in covenant obedience; but others, through unbelief, are not (1 Cor 15:45-47). Still in Adam, they are covenant breakers and despite being made in God’s image are at moral enmity with him (Rom 8:7; Col 1:21; Jas 4:4).
No matter where a man turns, God is there. If he looks outward to the created order he sees God. And if, as one made in God’s image, he turns inward, he again cannot escape God’s presence. As the Psalmist exclaimed:
Where shall I go from your Spirit?
Or where shall I flee from your presence?
If I ascend to heaven, you are there!
If I make my bed in Sheol, you are there!
Psalm 139:7–8 ESV
This is not merely the probability that there may be a ‘god’. It is the clear revelation that assails all our human faculties that the true and living God exists—that he is the Originator and Upholder of all that is. His divine nature, power, and attributes are clearly revealed to every person—believer and unbeliever alike. While this knowledge of God is not exhaustive, it is true.
For the unbeliever, then, to survive this overwhelming manifestation of God’s presence he must resort to mental speculations to justify his moral enmity with God. He must, as Paul explains, “suppress the truth in unrighteousness” (Rom 1:18 NASB), thus, becoming “futile in their thinking” (Rom 1:21 ESV) and “claiming to be wise, they became fools” (Rom 1:22 ESV). The unbeliever must contrive an alternative “reality”, believing a lie, in an attempt to deny his knowledge of God, particularly as the Originator and Upholder of all things.
This leads to worldviews and philosophies that seek to exorcise the universe of God’s physical and moral control. All unbelieving thought-systems are at heart an attempt to rationalize humanity’s would-be autonomy—that is, its independence of God and freedom of thought. Man’s mind now dictates reality. Man has become his own measure.
But unfortunately, a mind at enmity with God becomes futile. It becomes its own ultimate reference point, refusing to think the thoughts of God after him, and thus, misconstrues reality. By its own admission, the history of philosophy, therefore, is a failure to provide an intellectually coherent interpretation of the universe.
Because of God’s clear revelation to every person there are no atheists, only anti-theists. The believer, therefore, need not be an expert in philosophy to speak to the human condition; and can be confident that biblically grounded Christianity provides the only intellectually coherent interpretation of the universe.
This groundedness begins in God as the Originator of all things and ends in him as their Upholder.
Oh, the depth of the riches and wisdom and knowledge of God! How unsearchable are his judgments and how inscrutable his ways! “For who has known the mind of the Lord, or who has been his counsellor?” “Or who has given a gift to him that he might be repaid?” For from him and through him and to him are all things. To him be glory forever. Amen.
Romans 11:33–36 ESV