Why do people become atheists?
In conversations over the years, I have heard people describe their youth as attending Catholic school or being raised in a strict Christian home and so upon reaching adulthood, they quickly discard religious attendance and formally announce their non-belief in god. For these people, it would seem that institutional religion has soured their willingness to believe in a personal god.
Upon conversation with them, their initial atheism and hostility toward god and religion sometimes drifts into a less aggressive agnosticism. They might say: well, I don't know if there is something out there or not but it sure ain't what I was taught as a kid. Sometimes they might even get to the point of saying that religion and belief in god can be a force for good but they would simply rather not get involved.
Another type of objection I have come across is the "science has all the answers" perspective. These folks believe that god is created by man because it was somehow evolutionary useful to have belief in god. They will cite evolution as proof that god isn’t necessary. They will say there is no evidence for the existence of god.
One can point out that every culture has a concept of god. To which they say, see, that shows you god was created by man. To which one can counter: if there is thirst, there must be water; if there is hunger, there must be food; if there is a desire for god, there might be Someone actually out there. Freud's "wish fulfillment" argument actually cuts both ways. One can point out some of the weaknesses of evolutionary theory but usually that is met with well, give science some more time and they will figure out the problems which is a form of "faith". One can also point out the Big Bang Cosmology and ask them did god have anything to do with that? At this point, some might acknowledge a "deist" kind of god who wound up the watch (universe) and let it go. And others people may admit to a belief in a New-Age/Eastern type of impersonal god as part of the universe.
A third and probably most emotionally powerful and intellectually sticky objection is the classic: if god exists why is there evil in the world problem. Many people have either suffered a personal calamity or have had a loved one die an untimely death. The pain of their experience is as real as it gets. The emotional scars then harden into an intellectual rejection of the possibility god.
At an emotional level, what we (Christians) should offer is love, acceptance and support. And at the proper time, we can gently point out that God has chosen to enter into death, suffering and evil through Jesus. The God of Christianity is Jesus living amongst us and dying on the Cross. This God does not remain detached and separated from the messy fray of life.
At an intellectual level, Christianity holds the concept of "free will." A god of power could command obedience out of fear. But a god of love desires freely given devotion. Thus, god has allowed us free will and the price of that choice is the reality that life has evil and suffering.
Does this answer all objections for the atheist? Probably not. And to be honest, does it answer all doubt for the believer? Again probably not.
The final objection I have run across to god is simply: I want to be god.
No one actually says those exact words but the point is that the idea that I am accountable to a higher being is simply unacceptable. We want to be the ruler of our lives and be the masters of our domain.
And indeed, that is the message of the Eden story of Genesis. Adam and Eve had a choice to make. Do they take god's word for it that they could eat from any tree they wanted except for that one tree? It wasn't like they were going to starve to death if they didn't eat from the forbidden tree. So they choose.
Likewise, we choose every day. We can choose to live as if we are god or we can choose to live as if there is a God out there who cares about what we are doing.