Grounded in Reality
The [inward certainty resting on the Spirit] must be present if the Reformed emphasis on grace alone to the exclusion of works is to be maintained and paralleled at this crucial juncture, the doctrine of the self-authenticating authority Scripture to the exclusion of individual human proof and of churchly testimony. But the [external objective certainty resting on evidence] must also be present if the subjective conviction is to be grounded in reality.
Richard A. Muller, Post-Reformation Reformed Dogmatics: The Rise and Development of Reformed Orthodoxy; Volume 2: The Cognitive Foundation of Theology, 2nd ed. (Grand Rapids, MI: Baker Academic, 2003), 259.