Having thus surveyed in a broad sweep the message of the law, the prophets, the Gospels and the writings of the apostles in De Testamento, Bullinger summarizes his thoughts on the unity of the covenant on folio 25a. In the sub-section with the subtitle “The unity of the covenant” Bullinger states, “There is therefore one covenant and one church of all the saints before and after Christ, one way to heaven, and one unchanging religion of all the saints (Psalms 14 and 23)”. With respect to the era of the Old Testament and the era of the New Testament he points out that “The times are different, but not the faith”.
Bullinger also inserted a sub-section on “The source of the terms ‘old’ and ‘new’ covenant (folio 28b). After a brief consideration of Jeremiah 31:31,21, Ezekiel 36:26 and Galatians 4:24 he concludes, “…it is certain that the nomenclature of the old and new covenant, spirit, and people did not arise from the very essence of the covenant but from certain foreign and unnecessary things because the diversity of the times recommended that now this, now that be added according to the contrariety of the Jewish people. These additions did not exist as perpetual and necessary things for salvation, but they arose as unchangeable things according to the time, the persons, and the circumstances. The covenant itself could easily continue without them”. Taken at face value it may seem to indicate here that Bullinger is seeing a sort of hiatus between the period of Abraham and that ushered in by the coming of Christ with respect to the law rather than an understanding of progressive revelation.
(see 'The One and Eternal Covenant of God' in An Everlasting Covenant: Biblical and Theological Essays in Honour of William J. Dumbrell (Reformed Theological Review Supplement Series #4, 2010) pp201-233)