I happen to believe this. So, what do I happen to believe, you ask? That atheists, when they say they reject God, aren’t rejecting the living God because they can’t without first knowing God; and they can’t first know the living and real God without the Spirit; and if they had the Spirit they would be a Christian; but since they don’t have the Spirit they 
aren’t Christians; and thus have no capacity to reject the real God. They instead only have the capacity to reject a god who is really just a projection of themselves; no matter how many Christians they are surrounded by. Even if they can intellectually “know” about the God Christians claim to know, they themselves cannot make this claim since the Spirit is required to know this God; to have eyes to see and ears to hear His voice. Since they are not this capacious, they may well be atheists; but they are atheists only insofar that they are rejecting the gods that the philosophers and they themselves have projected. If my premise seems tautologous, it is; but only insofar as God is the beginning and end of the circle. Barth agrees with me when he writes:
The God whose existence or manifestness they doubt or deny is not God at all. And so too His absence, as they think they should assert it, is not God’s absence at all. In order to be aware of God’s absence they would first of all have to know God and therefore God’s revelation. All general intellectual difficulties and impossibilities respecting knowledge of so-called supernatural things assert nothing at all in face of the negation of all other knowability of God which is achieved by God’s revelation itself. God does not belong to those supernatural things which may be believed and asserted to-day, doubted and denied to-morrow. And so, the difficulties and impossibilities respecting knowledge of these things, which the sceptic and atheist fancy they should take so very seriously, have nothing whatever to do with the hiddenness of God for man or man’s blindness for God. The seriousness of the fact that God is not free for us, not to be possessed, first begins with the revelation which delimits this fact, yet also illumines and confirms it in its factuality.[1]
This has tacit relationship to Anselm’s fides quarens intellectum (‘faith seeking understanding’), but is also a bit distinct. Barth’s point here is more publically critical than that. It is more in line with Ludwig Feuerbach’s critique of cultural religionists who worship a god of their own self-projection; it is a constructively critical appropriation of that line of thought.
This has impact on a variety of things, one of which is the way we as Christians engage with non-Christians. As an evangelist it makes me think I shouldn’t be in the business of proving God’s existence to atheists or agnostics, but instead simply proclaiming the Gospel to them which is the power of God. Indeed, this sort of anti-natural-theological/law thinking kicks against the North American evangelical sub-culture in some stinging ways. But then, on the positive side, in the same sub-culture there is this sort of emphasis on simply proclaiming the Gospel to whoever will hear, and allow the seed to fall where it will.
Barth’s critique does indeed have implication towards the way the Christian theologian does their theologizing; no doubt. It is a matter of where the theologian starts their theologizing. Thomas Torrance and Barth were of a piece when it comes to this, even if the way they emphasized certain things made them sound a little different one from another when it comes to a natural theology. Nonetheless, they both are theologians of the analogia fidei or analogy of faith tradition; the tradition that grounds knowledge of God in God Revealed and then given to and for us in the vicarious humanity of Christ in and through the faith of Christ which is the basis for our knowledge of God. We can also pick up entailments of Calvin’s ‘faith as knowledge of God’ in both Barth and Torrance in this instance. These are important things that continue to run over the heads of many theologians in the current evangelical climate. They simply go on their merry-way, and act as if such things really don’t matter; they continue to engage in a textus receptus way of theology, wherein they simply see themselves as inheritors of a by-gone Protestant theology that represents, for them, the only genuine way to be an orthodox, conservative, evangelical theologian. But they are wrong. And more significantly, what is of ultimate import, beyond figuring out if we are in line with an ad hoc conception of who the orthodox are or aren’t, is to simply be focused on doing theology that is most proximate to the Gospel reality itself. In other words, who cares, ultimately, what the genetics are; the Gospel itself is the only genealogy that really matters.
Anyway, atheists, theologians, and all of us ought to be wary of thinking we can have a genuine knowledge of God apart from the revelation of God in Jesus Christ. We ought to start everything from that point or not start at all.
[1] Barth, CD I/1 §13, 28.