In his essay Calvin’s Covenantal Response, (in The Failure of American Baptist Culture) Peter Lillback writes concerning Calvin’s view of the law written on hearts by the Spirit. This summary is a wonderful treatment of the law against the Lutheran and neo-Lutheran:

Thus Calvin explains how one ought to compare law and gospel in his comments on Jeremiah 31:32ff. First, Calvin notes, one must recognize what the law is in itself–a rule of righteousness that only speaks to the ear as letter since it does not have the Spirit. But secondly, Calvin adds, this contrast ceases once the Spirit is joined with the law. It is then no longer letter, but actually spirit or the gospel itself. In fact, Calvin insists that it is not a new law that the Spirit writes on the heart, but the very same law that was once only letter. Therefore Calvin insists that the benefits of the New Covenant were even present in the law of the Old Covenant. To illustrate this, Calvin mentions John 1:17. If grace and truth have come through Christ and the law was of Moses, does this mean that these benefits were absent from the law? His answer is that even though grace and truth are only found in Christ, and the law doe not have them as benefits it can actually bestow, they were nonetheless present adventitiously. Simply, they were borrowed from the gospel. In light of this, Moses can be considered in two different senses. If he is considered without Christ in his narrow office (cf. comm. ad Rom. 10:4ff.) as lawgiver, his message was only letter and hence promised only death. But if Moses is considered in his whole teaching, he is seen to preach Christ as well. In that case, he must be considered as a preacher of the gospel, the same gospel as is found in the New Covenant.