A Puritan Theology: Doctrine for Life in a Year:
The covenant of grace is both conditional, for faith is required, and absolute, for the condition required by the covenant is also promised by the covenant. On the one hand, [William Ames (1576-1633)] said one cannot have certainty that he is saved by grace without “the perceiving of faith and repentance in himself” (1.30.16, The Marrow of Theology). On the other hand, to Ames, as John von Rohr points out, “the promise of fulfillment of covenant conditions was itself covenant promise.” In Ames’s words, faith, the condition of the covenant, is promised “to be given by grace as a means to grace” (1.24.19).
Thus, in the final analysis, grace does all, and the believer learns to rest on a promising, decreeing God.
(A Puritan Theology, p. 50)