You can't believe without the Spirit.
14 But a natural person does not accept the things of the Spirit of God, for they are foolishness to him; and he cannot understand them, because they are spiritually discerned.
That's why the Gospel is delivered wrapped up in grace. Grace is God's power to work His will, freely given from His merciful nature. Grace isn't simply a "warm fuzzy" God has but is strongly connected to His strength and power.
2 Corinthians 12:9
And He said to me, “
My grace is sufficient for you, for My strength is made perfect in weakness.” Therefore most gladly I will rather boast in my infirmities, that
the power of Christ may rest upon me.
The natural man is like a man born without eyes but the Spirit by grace acts like a surrogate and makes the truth known so that a man might give his own answer to God regarding His Christ. This is why even the most reprobate knows the truth because it is God Himself who manifests that truth to them even without the new birth. The irony is, you can't be an atheist unless you understand what the phrase "God exists" means at it's basic level.
Rom.1:18-23
18 For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men, who [
d]suppress the truth in unrighteousness, 19 because
what may be known of God is manifest in them, for God has shown it to them. 20 For since the creation of the world His invisible
attributes are clearly seen, being understood by the things that are made,
even His eternal power and Godhead, so that they are without excuse, 21 because,
although they knew God, they did not glorify Him as God, nor were thankful, but became futile in their thoughts, and their foolish hearts were darkened. 22 Professing to be wise, they became fools, 23 and changed the glory of the incorruptible God into an image made like corruptible man—and birds and four-footed animals and creeping things.
The call is to believe the Gospel for the Gospel is the power unto salvation. To say one must be saved (regenerated) in order to believe the word that saves you is circular to the extreme. We don't get saved to be saved.
Romans 1:16
For I am not ashamed of the gospel of Christ, for it is the power of God to salvation for everyone who believes, for the Jew first and also for the Greek.