Many people have pointed out that sequels are rarely as good as originals. National Treasure 2 wasn't quite as good as the original National Treasure. Don't even get me started about the 2nd and 3rd installments of Pirates of the Caribbean.
I think the reason this happens is that in the original, people are trying to capture the ultimate: best treasure-hunt movie ever, best pirate movie ever. Whether they succeed in being the best is one question, but their high goal leads to high results. Sequels often aim lower. They do not want to be the best ever; they just want to repeat the first. The same players come out to take another bow. They end up with the quality of stale leftovers, warmed over and not quite fresh.
The sequels that break this pattern are the ones that again seek to be the ultimate instead of being mere encores.
In the early days of Christianity, followers of Christ were breaking new ground. The territory was the whole world. Teaching monotheism to pagans -- or a loving God to those who sacrificed their children -- was a bold move. Love of enemies was a bold move, still original in the history of religion. Forgiveness on a radical level challenged the standard compromise of love with resentment. And above all, the living memory of Christ fueled the growth of Christian thought and teachings. The true "first quest for the historical Jesus" was the one that put away Marcion and the Gnostics as those who preached a non-historical Christ. The origin of orthodoxy was loyalty to the realities of Christ.
As much as I'm a fan of orthodoxy, an advocate of getting our teachings right, I have a caution: If we define "right teachings" in terms of Nicea, Constantinople, and Chalcedon, we become a sequel. Please bear in mind that this "sequelizing" happens even if every word from those councils should be proved true. We end by trying to hold on, not to the original revelation of God in Christ, but to decisions about what that meant. The age of the early theological giants of Christianity is largely the age of people who looked directly to Christ. By all means we should read them: but in order that with their insights we may surpass them in their knowledge of Christ, not that we should see them as the upper limit and so continue to fall short. The sequels that break the pattern of mediocrity are the ones that again seek to be the ultimate instead of being mere encores.