Calvin has the highest regard for the preaching of the gospel by the human minister of the word. In and by the preaching, God himself speaks, thus working the salvation of his own children. In the preaching, by the ministry exercised by ‘a mortal and despised man, …God himself appears in our midst.’ God himself speaks in the preaching: ‘He deigns to consecrate to himself the mouths and tongues of men in order that his voice may resound in them.’ The preacher of the doctrine of salvation is ‘his [God’s] own mouth.’
…Because the preaching is the living voice of God in Jesus Christ, ‘the church is built up solely by outward preaching.’ ‘God breathes faith into us only by the instrument of his gospel, as Paul points out… [in] Romans 10:17.’ ‘The power to save…God…displays and unfolds…in the preaching of the gospel.’ Calvin appeals to Romans 1:16. In the preaching God himself ‘comes[s] down to us, in order to be near us…[and by this earthly means] to bear us up as if in chariots to his heavenly glory.’
…..Nothing is attributed to the human preacher, however, for it is God who freely joins his Spirit with the preaching, and he alone accomplishes all the salvation worked by the preaching. The same Paul who ‘boasted’ in I Corinthians 4:15 acknowledges in I Corinthians 15:10 that all his work was ‘the grace of God which was with me.’
Taken from The Reformed Faith of John Calvin: The Institutes in Summary by David J. Engelsma (Reformed Free Publishing Association, 2009), 312-13.