We are reconstructing the gospel from the apostles’ writings. None of them wrote as for a time capsule. That is, none of the 27 documents of the New Testament present themselves as a catalog of their faith or a constitution of their belief. Nothing like the Law of Moses is found. Instead, we have four narratives of Jesus’ ministry, passion, and resurrection plus a variety of letters written for a variety of reasons, most dealing with some specific situation or situations in local church gatherings.
At many places in the New Testament we can find encapsulations of the gospel or aspects of the gospel (e.g. 1 Corinthians 15:1ff. and 1 John 1:5), but nowhere do we find a full exposition of all that they meant when they spoke of being called to “preach the gospel.” Therefore, we must do our best to reconstruct the gospel from all that we find in their writings.