Many, with good reason, look to John Calvin's Institutes of the Christian Religion (1536) as the first and chief systematic treatment of Reformed theology. But it is important to realize that he drew upon earlier systematic theological works by other Reformers, which are not to be forgotten.
Philipp Melanchthon is credited with publishing the first Protestant systematic exposition of theology in 1521, Loci communes theologici.
In 1524-1525, while in Montbeliard, William Farel wrote and published the first Reformed systematic theology in French, Summaire briefue declaration daucuns lieux fort necessaires a ung chascun Chrestien pour mettre sa confiance en Dieu et ayder son prochain.
Also in 1525, Ulrich Zwingli published Commentarius de Vera et Falsa Religione.
And in 1529, François Lambert of Avignon, the author of the first commentary on Revelation by a Reformer (1528), published Somme chrestienne.