In his influential article of sixty years ago, Trinterud concluded the following about Oecolampadius:
“….. As early as 1525 the Basel reformer Oecolampadius, in a commentary on Isaiah, had put forth the view that the eternal covenant of God with man was the law of love. This law was written on man’s heart at creation, and was only expounded by the written law of the Bible. To be blessed of God man must keep this covenant by obeying this law. Here the entire law-contract structure is seen. The relationship between the ruler and the subject is contractual, based primarily upon a natural law which, in turn, is applied by positive law which declares its meaning in specific instances” – Leonard J. Trinterud, “The Origins of Puritanism”, Church History, vol. XX (1951), p41.
Trinterud clearly sees Oecolampadius as viewing the covenant in terms of a bilateral pact or contract. The same view was espoused by Kenneth Hagan in “From Testament to Covenant in the Early sixteenth Century”, The Sixteenth Century Journal vol 3 (no 1), 1972, p23. Baker, however, argues that Oecolampadius views the covenant as a unilateral testament of grace while Bullinger has a contractual understanding of covenant. Be that as it may, I would argue that Bullinger did not view the covenant as a bilateral pact or contract.
It is worth noting, however, that Oecolampadius was the only reformer cited by name by Bullinger in his De testamento. Bullinger cites a section from Oecolampdius’s commentary on Jeremiah: “Before God, that eternal covenant which is arranged differently according to the diversity of the times is one. And also in relation to the inner realities, it always has been one and will remain one, not only as it is in eternal predestination… Notice, however, the great diversity of the covenants. The Lord made a pact with Abraham with words and demanded nothing except obedience from him. But under Moses many strange and dreadful things were added, things known not only to the one leader but things evident to all the people. Then it was fortified with so many circumstantial legalities, all of which return to those ten words of the tablet of the covenant.”
In a footnote, Baker indicates that Bullinger selectively cited this work of Oecolampadius to suit his own purposes: “As indicated in the text, Bullinger deleted several lines from Oecolampadius. Oecolampadius was not as supportive as Bullinger would have the reader believe. Although the passage quoted by Bullinger seems to support his view of only one covenant, the material proceeding and following, as well as the portion deleted, supports a two covenant scheme, old and new, carnal and spiritual, corresponding with law and gospel” (Fountainhead of Federalism, p 128, n23).
Despite the fact that Oecolampadius and Zwingli were very close it does not appear that Zwingli’s understanding was due to influence from Oecolampadius. There appear to be some similarities between Oecolampadius and Bullinger vis-à-vis the covenant but clearly there were differences.
For a careful study of Oecolampadius see the dissertation of Diane Marie Poythress “Johannes Oecolamadius’ Exposition of Isaiah Chapters 36-37” (dissertation, Westminster Theological Seminary 1992).