I stumbled on an article by A. T. Pierson on the unity of Scripture tonight. It was published in Volume 7 of The Fundamentals, an original set of which I was given over a quarter of a century ago by a family friend, now in glory. I wouldn’t be keen on his dispensationalism, but I did find the following paragraph towards the end of the article a rather engaging summary of the New Testament letters:
The Epistles are likewise all necessary to complete the whole and complement each other. There are five writers, each having his own sphere of truth. Paul’s great theme is Faith, and its relations to justification, sanctification, service, joy and glory. James treats of Works, their relation to faith, as its justification before man. He is the counterpart and complement of Paul. Peter deals with Hope, as the inspiration of God’s pilgrim people. John’s theme is Love, and its relation to the light and life of God as manifested in the believer. In his Gospel, he exhibits eternal life in Christ; in his epistles, eternal life as seen in the believer. Jude sounds the trumpet of warning against apostasy, which implies the wreck of faith, the delusion of false hope, love grown cold, and the utter decay of good works. What one of all these writers could we drop from the New Testament?*
There is a good deal more in the NT letters, but this looks like a useful overview.
* Arthur T. Pierson, The Testimony of the Organic Unity of the Bible to its Inspiration, in The Fundamentals: A Testimony to the Truth, edited by R. A. Torrey (Chicago: Testimony Publishing Company, 1909-15). Vol. 7, Ch. 4, p. 68.