Since I'm sounding off on the hot topics of the day I figured I'd lay out my opinion on this issue as well.
I don't think abortion is a Christian issue as it's been presented by both sides in the American debate. It's a philosophical one. It involves determining when a human life counts as a human life and deserves the rights afforded to every human. If you think an embryo qualifies as a human life, that is not necessarily based on religion, though your religion may teach that. It's a judgement call. If you think a fetus is not human enough to have rights one second before it's out of the birth canal, but then becomes human one second later, that's not religious. It's a judgement call. The reason I'm 'pro-life' has nothing to do with religion. It's because I don't think we have a right to arbitrarily decide what a human life is.
I think it's clear that what we VALUE in human life slowly builds as an embryo develops, and indeed, even after birth. It's a spectrum, not a binary state: where it's a clump of cells one moment, and full human life the next. That makes no sense to me. Nor does it make sense to have the will of the mother change the status of an embryo from non-human to human. You get that bizarre contradiction where someone can get charged with double homicide for killing a pregnant woman, yet if that same woman had an abortion there are no legal ramifications. The only difference being the will of one human determining the status of 'human' for another.
My personal opinion is that embryos, fetuses, and even new born infants are not really human in the way that most humans require to be worthy of individual freedom and rights. 'Humanity' is a spectrum, with comatose people on machines keeping them alive, profoundly retarded people and babies on one end, and beautiful, intelligent celebrities and doctors on the other. The older a child gets the closer they get to the side of the spectrum where we value their humanity and afford them the rights assigned with that humanity. (I do not share this valuation, I'm speaking of society in the aggregate.)
That's all very logical, in a Peter Singer sort of way. It also follows that chimps have more 'humanity' than infants. I can't really argue against that from a purely 'scientific' (using this in a colloquial definition) perspective. My problem with this view is not religious or spiritual or moral. It is utilitarian. A society that accepts and embraces this view of value attached to brain-capacity and usefulness-to-society will necessarily cannibalize itself of the foundation of values, including the ones that determine utility. So while I don't think a human baby is really "as human" as an adult doctor, I think we as a society HAVE to assign it that status or else we face a collapse of values in favor of utility. Which is ultimately self-defeating because 'utility' is not a value-free concept. (What is 'useful' is predicated on a purpose imbued by values.)
This is very similar to my take on free will. I don't believe that it exists. But I understand that in order for our social structure to stand, and in order for us to understand what we value, such as love, justice, etc. we must act as though free will does exist.
My philosophical problem with abortion is that it creates a precedent that says that the desire of one life DETERMINES the validity and imbued rights upon another life (or life-form, if you will). That precedent has implications in all sorts of areas. But mostly it breaks the UNIVERSAL (not Christian) Golden Rule of empathy that is the foundation of virtually every moral system in the world.
I also know that most people don't think this way, and are also told lots of things that obscure this philosophical side to the issue, so I don't blame women who get abortions. I don't think they are murderers. But I do think it is wrong and ought to be opposed for the sake of keeping the values that make society possible. I think that the more abortion, euthanasia and the like are accepted by a society, the less they are moored to the foundation of what we all agree is morality. But the decision that a woman makes to have an abortion is not an abstract intellectual or philosophical exercise, but an excruciating and stressful time with a constellation of pressures from many angles. So I can hardly stand in judgement over them and tell them they are being immoral or terrible people. I think abortion is a mistake, and it undermines the Golden Rule. But we all make mistakes, especially when we are undergoing excruciating and stressful times. And I could be totally wrong. And I've never been in anything like that position. This is why I'm not a crusader against abortion, but I still think it is harmful to society.
Here's a parallel to help illustrate my point. The natives on Easter Island cut down all their trees, and this led to the destruction of their society. Were I to travel back in time to before all the trees were cut down, and approach a wood-cutter, I would not be coming at them with righteous indignation making a moral judgement about the act of cutting down the tree. They felt it was necessary to cut down those trees in order to erect those giant stone heads to display the power of their tribe, to keep them safe, etc. Within the context of their culture and in their minds, they were doing what was best. My argument to them would be that while those things may be true, there is a truer truth, which is that every tree they cut is bringing them closer to losing what it is they hoped to save.