I don’t think this is the argument being made by religious opponents of abortion. They’re not making a religious argument; they’re making a moral one. They’re against abortion because they believe taking the life of an innocent human is wrong.
This does stem from a belief that all human life is sacred, which does have religious underpinnings. However, it isn’t necessary for you to ascent to those beliefs in order to believe that killing human life is wrong, nor are they requiring you to do so.