Zwingli, Bullinger, and Calvin all represented what we might call the earliest phase of covenant theology. They used the term “covenant” quite regularly, understood it to have two dimensions, one of them historical and the other eternal, and applied it to the sacraments, especially in defense of infant baptism. Beyond that they did not go, at least not in any systematic way, and so it would be anachronistic to say that they were covenant theologians in the sense that this came to be understood in the next generation. Yet, even in Calvin’s and Bullinger’s lifetimes, things were moving toward a more systematic approach which would make them look increasingly conservative and even old-fashioned in the wider Reformed context.
From: God Has Spoken: A History of Christian Theology by Gerald Bray (Wheaton: Crossway, 2014), p. 589.