What should Christians who accept evolutionary biology declare to be "essential" to an orthodox Christian faith? For me there are several essentials that if they were given up would evacuate the Christian faith of any meaning and substance. First off, that there is one eternal Creator God which created all that is, whether physical or spiritual. I would add that this creation was from nothing, otherwise known as ex nihilo. Yet I understand that other Christians differ on whether this belongs to the essential category. I put it there because of the nature of God being eternal and before all things, thus anything other-than-God must be created, and ultimately that means it came from nothing into being. Unless we hold to a kind of pantheism or panentheism which has the created world being part of God. My understanding of the biblical text is traditionally theistic whch sees a sharp divide between the creator and the creation.
Another essential to the faith for me is the centrality of Christ in salvation. Whether you hold to a strictly exclusive understanding of salvation (no one can be saved apart from a conscious faith in Christ as savior) or a more inclusive understanding (Christ is still the only way to the Father, but there are hidden Christians, or a second chance to choose Christ after death) is not my main concern. The biblical witness is clear that reconciliation with God is conditioned upon Christ's work on the cross and especially in his resurrection.
Which is a nice segue to another essential that if rejected leaves Christianity being nothing more than another moralistic philosophy that's essentially indistinguishable from other belief systems. Miracles. In particular the miracle of Jesus' virgin birth, his miracles throughout his ministry, and ultimately his resurrection from the dead. Paul, not to mention the other apostles, makes it clear that their faith was predicated on Christ rising from the dead and therefore defeating death itself. If this didn't happen then Christianity is just another ultimately meaningless philosophy, no better, no worse. Actually it would be worse, because it claims to be more and if that's not true, then it would be guilty of knowingly lying about one of the most essential questions in life: What happens after death?
And since I opened this with a reference to those who, like me, accept evolutionary biology as true, is belief in an historical Adam and Eve necessary to an orthodox faith? I happen to believe that they were historical people. But I don't believe they were the only humans around at that time. Thus I have no problem with the necessity of a thousand or more early humans to provide the genetic diversity that we see in the human genome. I see them as representative figures standing as federal heads over all of humanity, just in the same way that Christ is the federal head over all those who are under him. So I would say that Adam was historically real, first of all because the rest of the biblical writers assumed that to be so, especially Paul, but also because there is no reason to reject his historicity for scientific reasons if he's seen as a federal head as mentioned above. It's when he's seen as the sole progenitor of all humans that we run into serious scientific problems. Ironically that same literalistic reading of early Genesis also sees the same genetic bottleneck with Noah as Adam. because if the flood was worldwide (btw, I don't believe the flood was worldwide) and only Noah's family survived, then we're once again stuck with the same genetic problem as we saw with Adam. So Adam makes the cut in my scheme, though I'm not sure how essential he is to an orthodox faith. But at least in my reading, he's really important in light of what the rest of scripture says.
There are of course many other doctrinal issues, some more important than others, involved in the Christian faith, not to mention the myriad practical aspects to being a true Christian such as charity, mercy, purity of heart, etc. Christianity, while it is a cognitive faith, is so much more than that. And it isn't a true Christianity that is only cognitive without the moral life being lived out.
So is this enough? Are there other essentials that need to be included? Is it too much? What might you drop in my list as an unnecessary burden?