If you think the only argument against abortion is a religious argument, please examine the pro-life case more closely. While a religious argument can be made, it is not necessary. The pro-life argument stands or falls on biological facts plus philosophical/moral reasoning. It’s very simple:

(1) It’s morally wrong to kill innocent human beings without proper justification (the philosophical/moral premise)
(2) The science of embryology demonstrates that a new, distinct human being comes into existence at conception (the scientific premise)
(3) It follows from (1) and (2) that abortion kills an innocent human being
(4) Therefore, abortion is morally wrong

If you are going to argue for abortion, you’ll need to falsify one of the first two premises above. Will you deny the moral truth that it’s wrong to kill innocent human beings, or will you deny the scientific facts of embryology?