(Disclaimer) First of all, if I have failed in any way to frame this objection as fairly and as accurately as would please an atheist or skeptic, I would invite someone who shares this view to re-phrase it in a way that is more in keeping with the objector. My intention here is not to create a "straw-man" with which I can easily dispose (for I genuinely find this to be a challenging objection), but rather to tackle the argument at its most challenging. Otherwise, what would be the point of presenting it? Secondly, I am approaching the argument point by point, from most general, like, can one have a general belief in a Deity; to most specific, like whether or not a deep devotion to God is even possible.
As a purely rational human being (i.e. as a materialist) I demand evidence for things, which is why science seems more practical to me. In other words, I put my trust in things which can be verified and tested empirically, not in things which lie beyond it. Thus, it seems wholly irrational and impractical to base your life on something or someone that you have never met, nor are likely to meet.
First Point: To trust in the work of science is already to admit to a design and a pattern to things, the only real question is who or what is ultimately responsible for that mysterious "pattern". Whether you believe that the order itself is a consequence of a Creator, or that it comes from nothing at all, you nevertheless must have full faith that the method is logical, trustworthy, and consistent. In either case, we come to trust in it for a very simple and practical reason... it works.
Second point: Presumably at their best, both religion and science (if we must separate them in this way) seek to come to the knowledge of what is true and real. Neither one would suggest that they are absolutely able to exhaust all that can be known about the physical universe and beyond. Nevertheless, they both embark on this particular journey in order to uncover clues and hints that further illuminate this mysterious engine that propels everything in the universe. In light of this, it should be noted that the scientist and the religionist both base their belief in things visible upon those which are not visible (i.e. wherever there's smoke, we naturally deduce there's fire). In other words, the things that we do see, provoke immediate interest or speculation in the things which cannot be seen.
Third point: The materialist contends that life can be lived on purely materialist grounds. I would argue that not only is this impossible to live for those who wish to live in a humane world, but equally for those who simply wish to continue to practice science. Indeed, everything that makes life worth living (not to mention livable) is empirically unverifiable. From the thoughts we think, to the virtues that we practice, to the conscience that atheists prize, to our free will, to love, to dignity, freedom; even the scientific formulas and mathematical equations necessary to justify a materialist world-view cannot technically be observed unless humans represent them. Thus, for every bit of science that we do (or anything else which is important in this world), we must inevitably presuppose/admit to some sort of immaterial reality in order to draw successful conclusions about what is in fact material.
Fourth point: This part of the objection is- in my opinion- the most potent and certainly requires our fullest attention. Whereas the first part of the argument concerns itself with the question of science vs. religion (of which, again, there need not be opposition) the latter raises the question of whether or not we can really have a deep and abiding relationship with a personal being whom we have never met. The only way to answer this part of the objection is to see if we can observe instances of this in the world around us.
First of all, we are certainly not privileged enough to have met every single ancestor that is responsible for us being on this planet (including the first one), nevertheless we know logically without that "first cause" there would have been no effect (namely us). So in this one sense it is irrelevant whether or not we have met them. Even if we only knew this much about God, it should be enough to inspire some sort of reverence and gratitude (if only in a Deist manner), just as it is appropriate in the case of honoring our own ancestry, many of whom we have never met.
Secondly, it is also possible to compare this remote kind of devotion to the "celebrities" that we venerate in our culture. Once again, we may never have been blessed enough to meet them personally, but simply by reading their words, hearing their music, listening to tales about them, etc., we nevertheless feel that they have touched our lives in ways that perhaps even those closest to us have not. In all walks of life there are these heroes (dead and alive) that, for whatever reason, we seem hard-wired, or at least generally inclined, to venerate.
And then lastly, I offer the following analogy in an attempt to offer a rational justification for having a deep and profound devotion, nay a communion, with one whom we have never technically met. A child need not remember his father in order to know that he longs for his embrace. In the case of an orphan, the child is often haunted by questions surrounding the nature of his conception and birth, especially as it relates to his parents identity, as well as why they left him in the first place. So even in the absence of this parent, the child still finds himself affected by this person (even if it is only in the negative sense). This does not simply hold true in the human sense, but also spiritually. Where is God? And why did he abandon me? Am I an accident? Or was I conceived in love? From a Christian point of view (and to some extent a Jewish), we have an answer to this question; "Can a mother forget the child at her breast, even so I will never forget you" (Isaiah 49:15); "Even if my father and mother forsake me, the Lord will gather me in" (Psalm 27:10). This promise in the Hebrew Scriptures is fulfilled in Jesus Christ, for in the Incarnation we find out that the father's absence is not the result of abandonment, as was true in the case of his first parent (viz. the sin of Adam), but rather the result of heroism. It might likened to a father who dies in the process of saving his baby boy. And though the child remembers nothing of these events, the mother (i.e. the Church), consoles the boy as he grows up, telling him stories about his father's incomparable goodness. She also presents to him a diary addressed specifically to the child, which chronicles the events of his life, expressing, among other things, the father's deep and abiding love for the boy, along with advice on what it means to be a virtuous man. The "diary" ultimately ends with a promise, albeit one that requires tremendous faith, one that ensures a reunion of the two as long as the child follows in the father's footsteps.