The apostle means we the apostles, and the New Testament prophets. The Scriptures took over as the apostolic word was written down, the Scriptures were complete.
Supposing you read Hebrews chapter 6, a famous chapter of Hebrews. The first time I ever read it – and it is true with many people – I got a bit of a shock, because it gave me the impression that a child of God can lose his salvation. Ah but you see, when you compare Scripture with Scripture, you realise you have got it wrong, because other Scriptures that are unassailable and incontrovertible tell us about the perseverance of the saints, and the absolute security of the person who is truly saved. So I had understood Hebrews 6 wrongly. When I looked at it again more closely, I saw where I had gone wrong. I saw how I had read it only superficially; I saw it could not possibly say what I first thought it said. What corrected me was the principle of comparing Scripture with Scripture. It works this way also: if you take all the unassailable basic truths and doctrines of the word of God out of the Scripture, you can form a systematic theology. Have you studied botany? The botany textbook is about trees and flowers. It does not look at nature like we look at nature. We see a great profusion and variety of all kinds of flowers and plants, and it is beautiful. But the botany textbook just takes out individual families of plant, and puts them into a systematic order. If you arrange the garden like that, it would not look beautiful at all. But the botany textbook takes out all the families in an orderly manner, and you study each one separately. Now that is what we mean by systematic theology. It takes the wonderful profusion of teaching of the Bible and it organises that teaching under doctrinal headings. It gives all the texts but it focuses on one doctrine at a time. We will look at this doctrine, and then at another doctrine, until you have reproduced the Bible, hopefully, but in a systematic manner with all the doctrines set out one after the other. Now the Bible itself is much, much better of course for us. Systematic theology is not competing with the Bible, but it has a purpose. If you have all the doctrines summarised and set before you, then any understanding of the Scripture can be checked against the body of doctrine. Thomas Watson's grand puritan book on doctrine which everybody should read is called ‘A Body of Divinity’. You have got all the main heads of doctrine teased out and identified and listed in a systematic way. And this applies here: ‘Comparing spiritual things with spiritual.’ Check your understanding against the body of incontrovertible doctrines, so clearly taught, so many times, and it keeps you on the straight and narrow.
Every major error that is ever committed in Christendom among the churches is due to this principle not being applied. Somebody has gone to a text and said, it means this, and they have not compared it with the other texts on the same subject, and they have misunderstood it, and they have built a denomination on it sometimes, or whole movements, and misled thousands. Every error can be traced down to a failure of comparing Scripture with Scripture. So the apostle gives us here, a great rule of interpretation, so that we can be clear on what the Scripture actually teaches.