The abortion question doesn't hinge on religion, it hinges on when life begins. It just so happens that religion provides some people with an answer to that question.
If you have a fundamental belief that life begins at conception, that from that point it is a unique, human life, then, whatever the source of that belief, you have a duty to oppose abortion and it's reasonable to attempt to make illegal what you believe is murder.
Their position is not to impose their religion on others, but to oppose what they believe is wholesale murder -- again, it's a reasonable position given that belief.
Unfortunately, the answer to when it's a human life isn't answerable. Personally, I have issues with the extremes on both sides of the debate.
I think it's ridiculous to argue that it's a human being right after conception and hypocritical to say "well, okay, we'll let you murder that baby if you were raped".
On the other side are the proponents of abortion on-demand in the third trimester. Arguing that at eight months it isn't a human baby and has no rights is disgusting and reprehensible to me.
Again, unfortunately, we have no measuring stick for where that line between blob of cells and baby actually lies. The question I ask is:
Given that our entire legal system is based on the premise that it's better to allow the guilty to go free than to punish the innocent, that doubt should always fall to the possibility of innocence, shouldn't the maybe-child get the same benefit as the maybe-innocent-murderer?