In the lectures on Romans that he gave in1525, Bullinger stated that he differed from Luther and Melanchthon in some aspects of understanding the message of Romans. For example, with respect to ’εκ πιστεως ’εις πιιστιν (1:17) Bullinger concluded that Paul means “the righteousness of God emerges from faith in (his) faithfulness”. Thus πιιστις is first understood as ‘faith’ and then as ‘faithfulness’. What Bullinger meant by this faithfulness is the faithfulness of God that leads a person to faith. From the very beginning with his lectures in 1525, Bullinger underscored that fides encompasses ‘faithfulness’ both on the part of men and women as well as on the part of God. He concluded that the divine fides is identical with God’s ‘faithfulness’, His fidelitas and His veritas.
(cited from “Bullinger and Romans", Reformed Theological Review vol 69 (2010), pp34-47, p43)