Christians often have to wait for history to unfold before we recognize important biblical truths. Human rights is an example. The Bible does not use rights language, and so naturally, it would have been hard at first to grasp how the Bible could be a great, if not the greatest human rights document. But a closer look reveals some implicit biblical rights: The commandment to not steal assumes the right to property. The commandment to not kill assumes the right to life. The teaching that we are created in the image of God argues for the intrinsic value of human beings. And so on and so forth. That Christians now champion human rights across the globe and ground their efforts in their reading of Scripture—well, it simply illustrates the many levels of the Bible and its teachings. It's no wonder that this book remains a source of inspiration to billions, and no wonder that sometimes historical events have to unfold before we can see some of its deeper truths. That's how great literature works, and the Bible is certainly that, and for some of us much more.
That being said, it is fair to say that the Bible does not teach that "life begins at conception." We cannot find a verse that puts it so simply and clearly. But evangelicals are not wooden literalists. We firmly believe for example, that Jesus was "true God and true man," and that God exists as a "Trinity," "three persons" of "one being." None of that precise theological language is found in the Bible. But taking the teaching of Scripture as a whole and trying to understand what it claims about Jesus and God, we have found that this Trinitarian language a very good summary of what the Bible teaches.
In line with Dudley's historiography, a school of interpretation has arisen to suggest that the Trinitarian conclusions at the Council of Nicaea were driven not by theology or Scripture as much as church and imperial politics. Ramsey MacMullen's Voting About God in Early Church Councilsis a scholarly example, and Richard Rubenstein's When Jesus Became God: The Epic Fight over Christ's Divinity in the Last Days of Romeis a popular example. As one commenter on Amazon.com says about MacMullen's book, "So in the Christian patristic era, truth was determined by majority vote." Despite the cynicism, such books helpfully draw out themes that have been sometimes neglected by more pious historians.
While such histories do us a great service—reminding us of how God uses the messiness of history to accomplish his will—they do not really speak to the matter of providence or the validity of a particular theological truth. Christians--who appreciate the doctrines of sin, incarnation, and grace--are not surprised that the church has been rampant with politics and coercion, that many doctrines arose in particular times and places, and in rather ugly ways at that. What's remarkable is that God has continued to use this rather messy business to accomplish so much good.
Abortion in the Bible
The Bible teaches that God forms us in the womb (Job 31:15, Ps. 139:13) and affirms the value of human life ("You shall not murder"). The great tradition of the church has from the beginning condemned the evil of abortion; for example, The Didache, written in the first century, is one of dozens of early church documents that explicitly rule out abortion for believers. So, it's hardly a stretch for Christians to conclude that perhaps life begins at conception. Christians of good conscience can disagree about when exactly human life begins, but the vast majority of Christians, Catholic and Orthodox and Protestant, across long stretches of time (2 millennia) have said that abortion is a grave evil, and grounded that in their interpretation of the biblical command that forbids murder. The remarkable historical thing is not that evangelicals came to believe this, but that at one time many didn't believe it. One reason they didn't believe it was because, well, it was a Catholic thing. But once they began to appreciate the teachings and resources of this great tradition, they could see and appreciate those theological and biblical arguments afresh.