I wonder if Gabriel Biel is the first covenant theologian?
"Gabriel Biel's doctrine of justification is based upon the concept of a pactum (covenant) between God and humanity which defines the conditions which humans must meet if they are to be justified, as well as emphasising the divine reliability. The present order of salvation, although radically contingent, is nevertheless totally reliable and strictly immutable. Thus God, having freely and of his liberalitas determined to enter into such a binding contract with humanity, is now obliged to respect the terms of that covenant. God gives grace to those who 'do their best', precisely because of God's decision and promise to behave in this way."
pg 87 of Iustitia Dei by Alister McGrath