Having done campus apologetics for a long time, I speak from experience when I say not all people come to God through arguments that may be deductive (certain), inductive (probable) or by abduction. People are wired differently. Philosopher Clifford Williams argues that while humans may have desires and beliefs they fulfill through reason, they also look for emotional fulfillment, especially when it comes to believing in God. Thus, humans want both their reason and emotional needs satisfied. Williams lists several existential  needs that all humans desire: there is the need for cosmic security and meaning in life. Further, there is the wonder and awe of the cosmos itself. Humans also need the opportunity to love, to feel loved, and to be with the ones we love.
We have more desires and needs: to delight in goodness, to live beyond the grave without the anxieties that currently affect us, the deep-felt desire to be forgiven, and to obtain justice and fairness by ‘squaring things up’—hopefully in the next life. Wherever we go, we see masses of people yearning to grasp the existential needs so integral to our lives.
In a world without God, however, there is no ultimate meaning to life. Certainly, people can create their own meaning and purpose, as French existentialist philosopher Jean-Paul Sartre advises us to do. Again, if people can find meaning and purpose in relationships, careers, volunteer programs, or other things, what happens when these things are threatened or lost by factors that are beyond control? Thus, circumstances can shake people who are calloused towards God and cause people to recognize their ‘existential need’ for God. Now if God has created us with these existential needs, it would make sense that God is the only one who can completely fulfill these needs.
Of course, skeptics posit that these needs can be explained through biological and evolutionary processes, but this begs the question and assumes that Naturalism is true. It also reminds us of Freud’s position on theism, that belief in God is based upon fear. Freud believed that as people try to find meaning, hope, and the escape from death in this frightening, formidable world, they comfort themselves with the idea of a loving, Heavenly Father. For Freud, religion is simply wish-fulfillment.
Not so fast, though: Freud’s in error. He commits the genetic fallacy, which states that a belief is false due to its origins. According to Freud, belief in God is false because it is rooted in felt needs based upon fear, and not on rationality. But even if he is correct (that people believe in God out of fear of a meaningless, scary existence), this does not mean that God doesn’t exist. It just means some people believe in God for reasons rooted in emotions as opposed to rational argumentation. In addition, for Freud’s hypothesis to be correct, he would also have to know why every single person to ever have believed in God did so. That’s impossible.
Concerning our existential needs, atheist-turned-Christian C.S. Lewis wrote, “If we find ourselves with a desire that nothing in this world can satisfy, the most probable explanation is that we are made for another world.” In Christianity, God is the highest good, and he desires all human beings to be fulfilled in him; he also desires to know them, heal their brokenness, and delight in them personally in a mutual bond of love. In Naturalism, there’s just Nothing, ultimately.
For the record, I don’t think believing in God for only existential reasons is going to cut it in the today’s world. Instead, utilizing existential reasons as an apologetic needs to be part of an overall cumulative case for one’s faith.
 Clifford Williams, Existential Reasons for Belief in God: A Defense of Desires and Emotions for Faith (Downers Grove, IL: Intervarsity Press, 2011).
 C. S. Lewis, Mere Christianity (New York: HarperCollins, 2001), 136-137.