I'm not a bit surprised at your friends tenaciousness. After all, not only are they a believer, but they have also chosen to commit vast sums of money to becoming a student of the faith, for all that entails in committing to retain what others tell them that serves to re-affirm the veracity of that choice to attend as a student of the faith.
And as such, that student is told any in contention of the faith are an enemy of the faith and are damned and as such are to be guarded against because they seek company in corrupting the devout unto that damnation/way of thinking.
This is a means of insulating the faith to remain strong, against the adversary of fact/truth.
One simple test to find out if your friend is amicable to daring to think beyond the book, which will tell you if anything else after this test has a chance of getting through, is to set the stage of logical foundation so that your friend accepts that foundation before pursing the discussion of faith.
This question is certain to gain the attention of any believer that wants to hear you out when you try to debate the issues of the faith. Mainly because most believers believe agnostics (and any other NB), engage in discussion with believers because their soul knows it needs salvation and is strong enough to reach for it but still is weak in actually facing that truth without a fight. Consequently most believers are told it is their sacred duty to give every NB the opportunity to accept the road to salvation, by entertaining them in conversation that just might save their soul.
So, in the spirit of that, you might consider this;
Ask your friend if the bible tells us that mankind is a sinful creature in need of salvation. (They are, because the garden of eden insured they would forever be born into sin. Hence the need for salvation when they are accountable for their sins.)
After your friend says yes, ask if the bible is inspired by god.
Yes.
And the bible inspired by god was written by men? Yes.
Then how can that which is the infallible words of god, be infallibly translated through the sinfully corrupted consciousness of the scribes of man, since the words that god inspires are intended to save those who choose to believe the law of god
after it is written.
Then if you really want to hit the issue re-affirm with them what "Infallible" means. Then ask how old the King James Bible is. I.E. When it was published. (English translation published 1611) . Then ask if the KJB is the infallible word of god,how it is that that infallible word is a composite of the older Jewish faith, from the beginning in the Old Testament.
Many believers when asked something like this will say that Christians follow the New Testament. As if discounting the impact of the Old Testament, on such an inquiry that they simply can not respond to otherwise.
However, if this were true then Christians that only subscribe to the New Testament, are not abiding as Christians to the words
in the NT, when they deny the efficacy of the OT which foretold of the coming of the
Messiah(Jesus). Therefore if they claim they do not abide in any way with the Old Testament they are then refuting their own New Testament which speaks of Jesus
fulfilling the prophecies of the Old Testament). (*Not to mention refuting the 10 commandments which are the moral laws of the Christian faith!)
I wish you luck if you pursue this line of questions to your friend. However, realize that in their eyes
you are damned unless you see things their way. (I.E. accepting Jesus as your savior.). Anyone attending school to reiterate that faith so committedly, has already made their choice of what's true, despite what is fact. So if you do this with the intent of changing their mind, you are wasting your time. If your doing this to make them stop harassing you about needing salvation, your wasting your time trying to communicate that logically to someone that cleaves to faith (any faith) which by it's very nature is illogical.
Then you have to ask yourself the most important question when you consider your motive for this post. (I.E. countering your friend intent on having you see things their way.) Do true friends disrespect one another's faith? (Agnosticism is just as much a faith as any religious tenet).