Its a spiritual inability remember ? A carnal person can believe naturally, just like a devil can believe James 2
19 Thou believest that there is one God; thou doest well: the devils also believe, and tremble.
A devil and a natural man can believe in God, but its not a Gospel believing that pleases God and obeys God. See True believing requires a spiritual obedience and love for the Truth Rom 10:16
But they have not all obeyed the gospel. For Esaias saith, Lord, who hath believed our report?
can a devil do that ? No, neither can a person in the flesh because they cannot please God,or obey God.