The title of this sermon may lead one to expect Calvin to give a mystical spin on proper Christian living, what we today often call "godly behavior." But the sermon is actually an exposition of 1 Timothy 3:16 -- "And without controversy great is the mystery of godliness, God was manifest in the flesh, justified in the Spirit, seen of angels, preached unto the Gentiles, believed on in the world, received up into glory." Calvin takes this occasion to give a clear, orthodox presentation of the doctrine of the two natures of Christ.
Francis Schaeffer in How Should We Then Live? remarked that the Reformation was no golden age of Christianity, and Calvin here evidences that Christians in his time resemble Christians in our own. It is now as it was then: Christians don't know what they believe. That is, Christians don't know Christian doctrine, the teachings foundational to the Gospel. As Calvin notes, people are too occupied with their lives and fail both to read the Scriptures and to comprehend the truth of God as given in the sermons they hear. Indeed, how often do even faithful church attenders walk out the door and immediately forget the content of that day's sermon! Lunchtime afflicts us with a dulling amnesia. (Perhaps we should schedule the sermon earlier in the service.)
Calvin stresses the centrality of the doctrine of the two natures of Christ to the Gospel. Our atonement is secured in the righteousness of the one Jesus Christ, who is both God and man. As the righteous man, Christ sacrificed himself so that we might be justified, and he now mediates between God and us, as he is able to represent us, being of the same nature as we are. Yet as Christ is also God, we can trust in him. When Calvin quotes part of Jeremiah 17:5, it becomes evident that Calvin is not merely saying that Christ is trustworthy or dependable, but rather that he is more than just a man; observe Jeremiah 17:5a & 7 (NIV) -- "This is what the LORD says: 'Cursed is the one who trusts in man, who depends on flesh for his strength. . . . But blessed is the man who trusts in the LORD, whose confidence is in him." This passage reflects Calvin's frequent contrasts in this sermon between the power and majesty of God (particularly as revealed in Christ) and the weakness of man. All in all, Calvin invites us to meditate on the mystery of the Incarnation and of the two natures of Christ in order "to nourish our faith" and to avoid past heresies, which Satan has used to threaten the salvation message of Christianity.