How Could Someone in the Old Testament Have Believed without First Being Regenerated?

Questions about how someone in the Old Testament could have believed without first being regenerated and why the devil would need to blind anyone (as referred to in 2 Corinthians 4:3–4) if total depravity is true and people already aren’t able to choose God on their own.

• How could someone in the Old Testament (or any time period) have believed without first being regenerated?
• If total depravity is true in the sense of people being hardened and not being able to choose God without being changed prior to conversion, then why would the devil need to blind anyone as it says in 2 Corinthians 4:3–4?