Assuming that God exists, could the atheist's denial of God be reasonable?

I say Yes to the title question; Greg Bahnsen, glossing Cornelius Van Til, says No. 

Yet it should be clear even to the atheist that if the Christian God exists, it is 'reasonable' to believe in him. (Greg L. Bahnsen, Van Til's Apologetic: Readings and Analysis, P & R Publishing, 1998, p. 124, fn. 108, emphasis added.)

This is the exact opposite of clear. Atheists believe that there in no God, and thus that the Christian God does not exist, and the philosophically sophisticated among them have argued against the reasonableness of believing that the Christian God exists using both 'logical' and 'probabilistic' arguments.  So how could it be clear even to the atheist that if the Christian God exists, it is reasonable to believe that God exists?  Bahnsen's claim makes no sense.  It makes no sense to say to an atheist who sincerely thinks that he has either proven, or rendered probable, the nonexistence of  God that it is nonetheless reasonable for him to believe that God exists even if in fact, and unbeknownst to the atheist, God does exist.

Bahnsen is missing something very important: although truth is absolute, reasonableness is relative.  This is why an atheist can find it unreasonable to believe that the Christian God exists even if it is true that the Christian God exists.  Let me explain.

I do not need to spend many words on the absoluteness of truth. I've made the case numerous times.  Here for example. In any case, whatever  presuppositionalists  such as Bahnsen think of the details of my arguments, they will agree with my conclusion that truth is absolute. So that is no bone of contention between us.

Reasonableness or rational acceptability is something else again.  It is not absolute but can vary from person to person, generation to generation, social class to social class, historical epoch to historical epoch, and in other ways.  Let's quickly run through a few familiar examples.

1) Falling bodies. It 'stands to reason' that the heavier an object the faster it falls if dropped from a height. It's 'logical' using this word the way many ordinary folk often do.  Wasn't Aristotle, who maintained as much in his Physics, a reasonable man? But we now know that the rate of free fall (in a vacuum) is the same in a given gravitational field regardless of the weight of the object in that field.  So what was reasonable to Aristotle and his entire epoch was not reasonable to Galileo and later epochs. Rational acceptability is relative.

2) For the ancients, water was an element. For John Dalton (English chemist, early 19th cent.) it was a compound, HO. For us it is H₂O. Has water changed over the centuries? No. Truth is non-relative. What it is reasonable to believe has changed. Rational acceptability is relative.

3) Additivity of velocities. It 'stands to reason' that if I am on a train moving in a straight line with velocity v₁  and I throw a ball in the direction of the train's travel with velocity v₂, then the velocity of the ball will be v₁ + v₂. It also 'stands to reason' that this holds across the board no matter the speed of the objects in question. But this belief, although reasonable pre-Einstein, is not reasonable post-Einstein. Once again we see that rational acceptability is relative.

4) Sets and their members. Suppose S is a set and T is one of S's proper subsets. Then every member of T is a member of S, but not every member of S is a member of T. Now suppose someone comes along and asserts that there are sets such that one is a proper subset of another and yet both have the same number of members.  Many if not most  people would find this assertion a highly unreasonable thing to say.  They might exclaim that it makes no bloody sense at all. And yet those of us who have read Georg Cantor find it reasonable to maintain.  If N is the set of natural numbers, and E is the set of even numbers, and O is the set of odd numbers, then E and O are disjoint (have no members in common), and yet each is a proper subset of N which the same cardinality (number of members) as N.

5) When I was a very young boy I thought that, since I am right-handed, my right hand and arm had to be weaker than my left hand and arm because I use my right hand and arm more. Was that reasonable for me to believe way back then? Yes! I had a reason to hold the empirically false belief. Of course, my little-boy reasoning was based on a false analogy. If you flex a piece of metal back and forth you weaken it. If you a flex a muscle back and forth you strengthen it. Use it and use it up?  No, use it or lose it!

Examples are easily multiplied beyond all necessity. The point, I trust, is clear: while truth is absolute, rational acceptability is relative. What is true may or may not be reasonable, and what is reasonable may or not be true. 

What Bahnsen and the boys appear to be assuming is that both truth and reasonableness (rational acceptability) are absolute.   Well they are -- but only for God, only from God's point of view.  God is the IRS, the ideally rational subject. He knows every truth and he knows every truth without possibility of mistake. So for God every truth, being a known truth, is in accordance with divine reason, and everything in accordance with divine reason is true.  But we do not occupy the divine point of view. To put it sarcastically, only a 'presupper' does.

But of course neither we nor the presuppositionalists occupy the divine point of view. They only think they do.  But that conceit is the whole essence of presuppositionalism, is it not?