WCF Chapter 17, Of the Perseverance of the Saints, sits directly behind the sermon's opening observation. The threefold cord has not snapped because it cannot snap: those whom God has truly called cannot fall away completely or finally, not because their grip on him is unshakeable, but because his grip on them is. The confession locates the guarantee in God's unchanging purpose, Christ's continuing intercession, and the Spirit's indwelling presence. The growth Paul celebrates here is not the Thessalonians' achievement. It is God's faithfulness made visible in human lives.
WCF Chapter 5, Of Providence, speaks to the sermon's harder pastoral note. God as the great sovereign of all things is not a distant administrator of a clockwork universe. He extends his providence to the afflictions of his people, ordering all things, including suffering, for his own glory and for the good of those he has called. The confession does not promise that providence will always be legible from inside the valley. It promises that the God who leads his people through the valley is good, and that what he is building through it will one day be seen for what it is.