I have an atheist friend who likes to focus the debate on the fact that we do not know when life begins (no special focus on atheists; there are plenty of theists who go the same route). For the sake of argument only, I grant that point and respond: if you do not know when life begins, then resolve the doubt in favor of doing no harm. If you want to abort or to legally allow abortion on demand, then you, as an advocate of causing the gravest harm possible to human life, have the burden of proving that there is no life at stake. In contrast, if I oppose legal abortion on demand, then I do not have to prove when life begins--I merely say: if in doubt, let the baby come out--alive and kicking. Follow the first rule of the ancient medical profession: above all, do no harm (primum non nocere).
Dietrich Bonhoeffer, I think, would have agreed:
Destruction of the embryo in the mother's womb is a violation of the right to live which God has bestowed on this nascent life. To raise the question whether we are here concerned already with a human being or not is merely to confuse the issue. The simple fact is that God certainly intended to create a human being and that this nascent human being has been deliberately deprived of his life. And that is nothing but murder.
Quote from Bonhoeffer's Ethics, found in the above pictured biography at p. 472 (emphasis added).
Bonhoeffer is bold, but then again he is the same man who confronted Hitler and who paid for it with his own life shortly before the end of World War II.