The pro-life position entails that since unborn human beings are full human persons at fertilization, if you kill an unborn human being at any point in his/her development, you are committing an act of unjustified homicide which should be forbidden by law. Of course, many pro-choice people believe, not having actually listened very closely to the pro-life argument, that the pro-life view is grounded only in a religious belief. So they respond that we cannot legislate a religious point of view into law.
Of course, they are correct. But what they miss is that we are not trying to force people to become Christians, or worship Yahweh, or pray the Lord's Prayer three times a day. If the pro-life argument is correct, that human beings are full human persons from fertilization, then the law of the land can reflect that. As I've heard LTI's president Scott Klusendorf mention in a debate against Malcolm Potts, the law does not have to take a position on the soul to make murder illegal. If the unborn are full human persons as adults are full human persons, then the law is justified in making abortion illegal, just like it makes infanticide illegal, and just like it makes murder of older people illegal.
Consider a Venn diagram with two circles that intersect (hopefully my mathematical friends will be proud of me). In the left circle is the set "God's laws," and in the right circle is the set "humanity's laws." In the middle is the intersection between the two. We are not trying to legislate something that is merely God's law into the law of the land (again, we are not trying to make people follow Christian rituals or worship Yahweh). We are legislating one of God's laws into the law of the land, both of which intersect. Murder, rape, and theft also oppose God's laws, but they are laws that we should also institute into the laws of the land. Abortion is no different, as it is the intentional killing of an innocent human person.
The pro-life position naturally entails that abortion should be illegal. If the pro-life position is correct, then our government has no moral choice but to make the act illegal.