I’m trying to figure out exactly what this means:
It is specifically through the writings of the New Testament that we can be in touch, by means of critical investigation and analysis, with the apostolic witness to the gospel. It is from this gospel, recovered from Scripture but not identical with it, that the believing community derives its self-understanding as the Body of Christ and its understanding of what is appropriately Christian.
If I am reading this correctly, it is saying that hiding in the New Testament texts is the apostolic witness to the gospel. To get to that witness we need the tools of critical scholarship to dig through the layers of clutter and distortion. It is from that witness that we must further refine and reveal the actual gospel, which the apostles bore witness to but may not have fully understood or clearly presented.
I’m not sure that is the full meaning of what the author is trying to say.