One of the characteristic features of the writings of Bullinger was the emphasis on the unity of the Old Testament and the New Testament. He saw the theme of the covenant linking together all of the books of the canon.
Bullinger wrote the following in his De scripturae negotio:
“In brief, I find that the New Testament is nothing but the interpretation of the Old. Except that I saw that the Old promises, the New teaches what has been exhibited; the Old is more concealed, the New is more revealed openly; the Old has to do with veils and figures, the New with clear evidences and the very things themselves.”
(Heinrich Bullinger Werke Dritte Abteilung: Theologische Schriften – Band 2: Unveröffentlichte Werke aus der Kappeler Zeit (Zürich: Theologischer Verlag Zürich, 1991), p25)