The first thing to understand is that some people don't believe in God simply because the topic has never come up in conversation. This was the case with me. Supposed proofs of the existence of God might as well be spoken in an unknown tongue to such a person. Much better would be to probe the implications of a person's unbelief with questions rather than directly challenging people as if trying to score points in a debate. In fact, the real audience for conventional apologetics appears to be professed believers who need some reassurance or at least some way of accounting for how they came to believe the things they now profess. I have also found that a non-confrontational approach in a predominantly non-Christian context will sometimes invite people to seek me out and ask me what exactly I believe and why. The following is how I respond:
We might suppose that there is no God or general organizing principle, but then what is the basis of principles as Justice and Mercy? An act of self-sacrificial kindness and an act of gleeful cruelty might be equally be judged good or bad regardless of context if nothing bears testimony to our existence. Indeed, one might argue (as do devotees of Ayne Rand) that Good is entirely based on what is gainful to the Self and that Evil is any focus away from the Self that might lead to loss of advantage. One similarly conversely argue that Good and Evil are a function of what benefits the community as a whole, but that is merely the previous option writ large and as such would validate brutality against other communities as easily as co-existence depending on the mood of the community. From there we might extend the frame to Humanity, Earth, the Cosmos, or even simply immutability--at which point we enter a metaphysical realm that differs from religion only in having no authoritative basis apart from that which any individual or community might propose, which brings us back to where we began.
There is also the question of whether any of this matters in the long run after the sun explodes or burns itself out. Most people would accept that nothing would "matter" absent someone to whom something might matter, but most people probably also aspire to leave behind some sort of legacy (if only a memory or two) of there having passed this way, which is qualitatively like hoping for an afterlife--at which point we once again enter a metaphysical realm that differs little from religion. There might be some for whom no sort of afterlife matters, not even a bit of graffiti, but even they likely bristle at injustice, however odd their conception of justice, and feel compelled to respond appropriately according to their considered or momentary standards. None of this makes any real sense apart from evolutionary adaptation, but that is hardly a sound basis for establishing anything approaching universal principles. Some creatures are known to risk their own well being for the sake of entirely unrelated creatures (and sometimes even erstwhile prey) for whom they display empathy, but others are known for extreme violence against even their own mates or offspring.
The point is that insistence on meaning, justice, and mercy presuppose something (or someone) transcendent. On a less philosophical level, many find it difficult to gaze up at the heavens at night (especially in regions far from artificial light) or at some towering formation or tropical extravagance of exquisite beauty without intuiting a vital and personal presence as the source of these wonders. One might of course dismiss such fancies with the assertion that a transcendent God who truly desired that we know him would do better to make himself known to us, but to this I reply that such would not make any difference as will indeed be conclusively demonstrated one day. The fact of the matter is that those who come to faith either imbibe the habit from their parents at an early age or to later choose to believe as a way of reconciling disparate realities. In my case, I provisionally committed myself to belief in God on a dare to see if that would actually make any difference to how I made sense of the world. As it turns out, I found it impossible to reverse course for long.
However, two things have caused me to have serious doubts at one time or another: the first being incongruity of a cognizant and loving God with the observation of unspeakable human suffering and cruelty; and the second being the incongruity of the conduct of professing Christians with their purported commitment to and communion with a just and loving God. For me the latter was somehow far more disturbing than the first and has taken me much longer to process, but I have come to think of Christians as works in various stages of progress and the pace of their transformation a reflection of their degree of surrender to God. I think of the transforming work of the Spirit of God as so many dandelions in various stages of development stretching over hills and valleys with a cumulative certainty of establishment and fruition.* My understanding of the mystery of suffering and cruelty is that (bear with me) their function is to instill a profound sense of empathy for others and a profound sense of trust in God, which correspond to the two great commandments that underlie the two tablets of Mosaic Law. The more counterintuitive of the two functions is trust in God, which has two aspects: the main one being that this will somehow make sense and the second being that everyone will be somehow adequately compensated or recompensed.
Should a person come to believe in God after one fashion or another, one might ask whether God is any more just and loving than we ourselves. Any God so feeble as to barely exceed our own capacities surely could not have accomplished the wonders we daily observe even despite their degraded state. On the other hand, the proposition that God not only exists but is moreover truly just, truly loving, and intimately familiar with us would raise several thorny questions. Would not Justice imply Judgement, and would not Judgement imply Condemnation unless a significant degree of fudging were involved? Conversely, would not Love imply Mercy and would not Mercy imply Forgiveness? The resulting circle seemingly cannot be squared without compromising Justice or Mercy or both at once, which is exactly the rock on which religion founders. However this is also precisely what lies behind the incarnation, life, death, resurrection, and glorification of the Son of Man.
God prepared the way for his coming by raising up a nation of priests charged with proclaiming God to the world and bearing testimony over centuries of just how humanly impossible it is to meet even a modest semblance of God's standards of purity (the Mosaic Law) and just how seriously the consequences of any defilement, namely being cut off from the community and even from life itself. The Law decreed that transgressions could not be expunged except by death of the transgressor because only death dissolves all bonds, which of itself is not particularly good news because no one is innocent of transgression and therefore marked for permanent death or worse. Against this stark backdrop, God promised to take upon himself the task of judging and punishing our sin while also showing mercy and redeeming us in the person of an anointed one, the eternal God (beyond time) embodied in human form (eternal but within time) who plainly declared and literally embodied the heart and mind of God but was condemned to death as an enemy of the very God he proclaimed.
Jesus was himself not a transgressor but became one by receiving into himself those who surrender themselves to him as their Lord. Jesus thereby became responsible for the sins past and future for those who entrust themselves to him, and his death expiated our transgressions by taking them unto himself--something so profoundly disturbing that Jesus bled from anguish at the prospect. However, his death would avail us of nothing if he remained dead because death dissolves all bonds--including Lordship over anyone--which is why our justification before God and our resurrection in glory are entirely dependent on the resurrection and glorification of Jesus. We meanwhile continue to transgress and die, but our justification in Jesus renders that death as it were unlawful. We aren't simply immediately transported to heaven though because we have work here as ambassadors of the word of hope, and we are also learning through suffering (whether our own or that of others) to trust God and have empathy for others, thereby equipping us for an unspecified purpose in the world to come that will likely resemble our original purpose of promoting abundant life with a place of honor for each creature, plant, and even terrain.
There was a time that I listened to various tellings of these matters with bafflement; but, as I studied the Gospels for myself, the words of Jesus struck me as words of Life; and, believing him indeed raised from the dead by the same divine power that brought forth the cosmos, I entrusted myself to him as my Lord and my Savior. I have since diligently sought the face of God in scripture to lead me through this present darkness.
* Americans are notoriously averse to dandelions, but they are beautiful to behold (especially in large numbers) and they are both edible and nutritious in all their parts excepting the seeds. More importantly as an image of Hope, they are impossible to eradicate.
