What does abortion have to do with the existence of God? Not immediately apparent. Lots of believers feel that abortion is morally acceptable at least in some circumstances. The circumstances vary widely, of course. There might even be some atheists who believe that abortion is always morally unacceptable, because it violates the principle of empathy, which is a common (though fundamentally flawed) basis for atheistic morality.
However, these distinctions, and in fact the entire abortion debate, tend to gloss over the most important question: what would YOU do when faced with the prospect of an abortion? Moral questions answered in the abstract do not always come out the same as moral questions that become personal. If you are a woman who is pregnant but doesn't want to have the child, would you have an abortion? If you are the man who made that woman pregnant, would you condone and embrace that decision? When we talk about abortion in the abstract, we tend to overlook personal moral intuitions. I believe there are many people who say they support abortion rights but who would never have or condone an abortion themselves. We are seeing something important here, about God. It's one of the standard proofs of the existence of God: the ethical argument, or the argument from conscience.
It goes like this: human beings have an innate sense of the morality or immorality of certain acts. This innate sense we call the conscience. Although necessarily a part of ourselves, it is an ill-fitting part. My conscience sometimes seems to stand as a witness against me. This tends to prove that it did not arise spontaneously from within us. Rather, it is evidence of something outside us that is supposed to direct our actions. This we call God.
If P happens then Q will follow.
Running an irrigation system causes my garden to grow.
Therefore, my irrigation system must be running.
The flaw in the argument is obvious: the garden might be watered by rainfall, or underground springs. This is the fallacy of affirming the consequent: the fact that one cause has a particular effect does not mean that it's the ONLY cause that has that effect. Many atheistic arguments about God are stuck in this fallacy. If those atheists were better at critical thinking, they'd make more sense. But this is true of everyone.
In the atheist worldview, the fallacy is applied like this to the argument from conscience.
Social conventions form people's moral judgment.
Therefore, moral judgments are a product solely of social conventions.
You can substitute the words "evolutionary effects" for "social conventions" in the syllogism and see the same results. As with the garden example, moral judgments might have other sources. The point is not to PROVE that moral judgments have other sources, but only to prove that the atheist argument is illogical. And once you strip atheism of its pretense to logical validity, it immediately begins to fall apart for other reasons.
Take moral choices. The plain fact of human experience is that certain acts have moral content. We are marked with the moral effects of our decisions, whether we want to be or not. Read Crime and Punishment. No matter how many utilitarian justifications Raskolnikov stacked up, no matter how he steeled himself to do the act, it tore a hole through him. Talk to a friend who's had an abortion and who will be honest with you and with herself. I have a friend who once had an abortion, in her early twenties. She was a frank and uninhibited person, staunchly liberal. But when she shared that experience, years after it happened, she was still shaken. She had been young and single, the father was running for the hills, she was afraid. So she did what she did. It made all the sense in the world. She had the right. But, she told me, she cried for three days, and she swore she was never doing that again. I believe her.
The innate sense of right and wrong is like the innate sense of God. We can separate ourselves from the sense of God, just like we can separate ourselves from our sense of right and wrong. But this is not progress, or enlightenment. It is a disaster. When masses of people do this, when they fully embrace the view that morality and God are mere social constructs, they do not find themselves in utopia. The world supplies them with Stalins, and Hitlers, and Maos.
The Catholic philosopher Blaise Pascal wrote that there is a God-shaped hole in the heart of every person. This is not a statement about what we learn from logic. This is a statement about what it means to be human.