Alvin Plantinga defends against the Problem of Evil by saying that it may not be possible for God to create a world where people choose good of their own free will.
He claims that people can choose good of their own free will, and that a world where people do that is perfectly possible in theory, but , paradoxically, an all-powerful God cannot create such a world.
Plantinga's argument runs as follows :-
Suppose there are 2 possible worlds , World 1 and World 2, identical up to a moment in time , T, at which time a person makes a free will choice in the situation he finds himself in in both worlds. He can choose between ,say, to drink tea or to drink coffee.
The person has free will, so suppose in World 1, he freely chooses to drink tea, and in World 2, he freely chooses to drink coffee. Remember , these worlds are identical up to the time the person makes his free will choice. Plantinga's definition of free will means that a person can choose one way or the other in identical situations.
Plantinga then introduces an axiom, which he just assumes to be true.
He says let us suppose that , when a person finds himself in the situation he found himself in world 1 and in also world 2, he always freely chooses to drink tea. Let us give this situation a name and call it situation Tea/Coffee Choice.
Plantinga then says that God cannot create world 2, where the person would choose coffee, because the person would choose to drink tea instead.
Plantinga says that world 2 is a perfectly possible world, because libertarian free will means that if a person is in the situation he finds himself in world 2, he can freely choose to drink tea or freely choose to drink coffee, but it is a fact that he will freely choose to drink tea, so world 2 cannot be created.
Of course, Plantinga has contradicted himself here.
To take one obvious error, the two situations are not identical at all. In one situation, God has infallible knowledge that the person will drink tea, In the other situation, God has infallible knowledge that the person will drink coffee. That is quite a huge difference for circumstances that Plantinga claims are identical.
There are other contradictions in what Plantinga writes.
He has defined 2 worlds as worlds where situation Tea/Coffee Choice applies, and defined one world as the world where coffee is freely chosen by the person and the other world as the world where tea is freely chosen by the person..
Therefore, by Plantinga's own definitions, the claim is false that whenever Tea/Coffee Choice applies, tea is freely chosen.
Plantinga's claims are as follows
1) In world-1 , when Tea/Coffee occurs, tea is chosen
2) In world-2 , when Tea/Coffee occurs , coffee is chosen
3) Whenever Tea/Coffee occurs, tea is chosen.
These 3 claims contradict each other. This explains why Plantinga has never given any evidence that 3) is true or ever attempted to prove that it is true. It is hard to prove something is true, when you have defined it as being false.
Let me illustrate by applying Plantinga's logic to geometry.
Euclid's 5th Postulate supposes that if you have a point and a straight line, only one line can be drawn through the point that is parallel to another given line.
This seems obviously true, but what seems obviously true is not always true.
Surprisingly, it is logically consistent to say that there is no parallel line, and you can make a logically consistent world where there is more than one parallel line.
If Euclid was right, then the angles of a triangle add up to exactly 180 degrees.
However, let us use Plantinga's logic to show that it is logically possible for there to be worlds where the angles of a triangle add up to more than 180 degrees, but that not even an omnipotent God can create such a triangle.
Define 3 possible worlds, all having an identical point and an identical straight line.
It is logically possible that you can draw 0, 1, or more than one line through the point, parallel to the straight line.
World 1 has 0 parallel lines. World 2 has 1 parallel line. World 3 has more than 1 parallel line.
This means that it is logically possible for the angles of a triangle to add up to less than 180 degrees, exactly 180 or more than 180.
Now suppose that whenever this point and line occur in any logically possible world, Euclid was right and you can only draw one line parallel to the original line.
This means that you can only create triangles where the angles add up to exactly 180 degrees.
So it is logically possible for the angles of a triangle to add up to more than 180 degrees, but not even an omnipotent God can actualise such a world.
The fallacy is , of course, that you cannot say that Euclid was right in every logically possible world, as you have defined some worlds as being worlds where Euclid was wrong.
Euclid was a genius and he was quite correct to say that if his parallel line axiom was correct, then it is logically impossible for the angles of triangles to add up to anything other than 180 degrees. Euclid knew better than to claim that the angles of a triangle could logically add up to more than 180 degrees, but that such triangles can never be created. Euclid started with axioms and then worked out what was logically possible, given those axioms. He did not start by working out what was logically possible, and then introduce axioms later. All of his proofs of what is logically possible depend upon the axioms he has stated he is using before he gives his proof.
Perhaps Plantinga could learn from Euclid and state that if Plantinga's axiom is correct , that people always freely choose the same way in a certain situation, then it is not logically possible for them to choose differently in that situation.
There is a nice exposition of Plantinga’s argument at Link
This confirms that Plantinga’s argument depends upon contradicting his own definitions.
It says :-
Now consider the following: Suppose that there is a maximal world segment S* that obtains in W* such that Curley always does what is right in W*:
a. S* includes Curley being free with respect to A but does not include his performing A or refraining from A,
b. S* is otherwise as much like W* as possible, and
c. If S* had been actual, Curley would have gone wrong with respect to A.
This means God could not actualize W*. If he did, then he would have to actualize S*. But then Curley would have gone wrong with respect to A.
If God makes Curley do right by A, then Curley will not be free. Thus, God cannot actualize W*.
This confirms that Plantinga’s argument depends upon defining a world where somebody freely chooses right, and then assuming that people do not choose right in that world. By definition, that assumption is wrong.