But knowing the law is not enough. Because of the
internal, existential problem of human sin, Augustine argued that the internal
work of the Holy Spirit is also vital. It is the Spirit who writes the law on
the heart of the sinner, something for which Augustine sees obvious biblical
justification in Jeremiah 31:33 (On the Spirit and the Letter 19.33).
Augustine draws this out dramatically in his work On the Spirit and the
Latter when he compares the Israelites gathered at Sinai to the disciples
gathered together at Pentecost:
Now, amidst this admirable correspondence, there
is at least this very considerable diversity in the cases, in that the people
in the earlier instance were deterred by a horrible dread from approaching the
place where the law was given; whereas in the other case the Holy Ghost came
upon them who were gathered together in expectation of His promised gift. There
it was on tables of stone that the finger of God operated; here it
was on the hearts of men. There the law was given outwardly, so that the
unrighteous might be terrified; here it was given inwardly, so that they might
be justified. (On the Spirit and the Letter 17.29)
Here the Spirit transforms both the sinner and the
law by internalizing the latter. “Command what you will, and give what you
command!” That intuitive cry in the Confessions now finds its theological
expression in Augustine’s understanding of God’s grace, here focused on the
word of the Holy Spirit in writing the law on the hearts of sinners. It is this
work of the Spirit that makes the external law able to address the most fundamental
problem of humanity, the internal and psychological consequences of sin. The
Spirit works subjectively in the life of Christians to renovate them and to
bring them back to the path of true freedom. (Carl R. Trueman, Grace Alone
Salvation as a Gift of God: What the Reformers Taught . . . and Why it Still
Matters [Grand Rapids, Mich.: Zondervan, 2017], 80-81)
