Motivating Plantinga's Distinction between De Jure Objections and De Facto Objections

De facto objections target the truth of Christian belief.  De jure objections target the warrant of Christian belief, i.e., whether an agent can hold Christian belief in a rational way despite whether Christian belief is true or not-- for example, a person may believe that a dice will land on six, and it may be true that the dice will land on six, yet that person may just be lucky in having the right belief, thus having no warrant for so believing.  

Plantinga holds that his account in Warranted Christian Belief has shown that no de jure objections are separate from de facto objections to Christian belief.  That is, you cannot object to the rationality of Christian belief without having arguments that show Christian belief to be false.  

De jure challenge to Christian belief: An agent must have an argument for a target belief p to be warranted in accepting p.

Consider the following, call it C.

C: If Christianity is true and Reformed Epistemology is true, then Christian belief would not need argument to be warranted.

Let’s say that C is a true conditional in both 
(w0) the world in which Christianity is true,
as well as 
(w1) the world in which Christianity is false.  

In (w0), Christian belief is warranted on C.
In (w1), Christian belief is unwarranted on C.

Yet, the only reason that in (w1) Christian belief is unwarranted is due to the falsity of Christian belief and not because of its lack in arguments.

Yet, if C is true, skeptics in (w1) (as well as in w0) can only attack Christian belief in their world using de facto arguments, i.e., they must use arguments purporting to show that Christianity is false.