Thumping the Bibile at a secularist over abortion only makes one look like a kook. He simply does not understand. That is what Mike is saying. Evangelism needs to be done carefully to be effective. "To the Jew is am a Jew, to the Greek I am a Greek."
If a Christian or a Jew does not base his arguments on Biblical principles, which does not necessarily mean quoting specific verses in Scripture, on what does he base his arguments? There is natural law, but someone who interprets natural law as, say, B.F. Skinner did would be entirely at odds with someone whose understanding derives the writings of Thomas Aquinas.