It's not about anything being hard for God.
If He holds people responsible for what they won't believe, that must mean that He - being just - has indeed given them the ability to understand and to believe...or not...else He lies and His condemnation of them is unjust.
Since He is no respecter of persons, if He's given that ability to some, then He's given it to all.
There are many who WILL NOT believe. That is a fact.
It is also a fact that there are none who CANNOT believe.
There is no other way to view it in light of what the Bible actually says about Him.
And you know what that means.