Pastor Al: You need to reckon with the fact that we are living in a post-Christian culture. You can't appeal to people on the authority of the Bible because they don't recognize the Bible as authoritative. You can't tell them “abortion is murder; repent of it”, because to do so is to argue from a religious source they reject. You have to meet people where they are. You have to argue from secular science (which they respect), not from Scripture (which they don't respect).
Mr. H: The Bible's moral precepts are written on the heart – i.e., on the conscience – of every man. Therefore, when we say even to those who reject the Bible, “abortion is murder; repent of it”, although they may not like it or respect it, nevertheless they cannot escape its authoritative claims upon them, for it is indelibly inscribed in their soul by the hand of their Creator. They can't avoid that. So when the Word is preached even to a post-Christian culture, the truth resonates in their hearts, and conviction is brought, even if they've never read a syllable of Scripture.
Bottom line, you convince a post-Christian culture of the wrongness of abortion and their need of a Savior the same way the apostles convinced a pre-Christian culture of their sin and need of a Savior: not with dependence on science or popular consensus, but by preaching the moral precepts of the word of God written not only on parchment scrolls, but on every human heart” (Rom 2:14-15).
