Chapter 18.2

ii. This certainty is not a bare conjectural and probable persuasion, grounded upon a fallible hope; but an infallible assurance of faith, founded upon the divine truth of the promises of salvation, the inward evidence of those graces unto which these promises are made, the testimony of the Spirit of adoption witnessing with our spirits that we are the children of God; which Spirit is the earnest of our inheritance, whereby we are sealed to the day of redemption. 
The Confession employs both affirmation and negation to characterize the assurance of faith. Assurance is not a matter of guesswork or probability--a hope that may turn out to be disappointed. On the contrary, the certainty of our salvation is "an infallible assurance."  

Rather obviously, such a high degree of confidence cannot be based on something we find solely in ourselves. We are prone to doubt, and our propensity to sin sometimes makes it hard for us to be totally sure that we are saved. 

Providentially, the assurance of faith is based on God and not on us. Our certainty of salvation is founded securely on divine promises of salvation, as we read them in the gospel.

To be sure, we do find "inward evidence" that we are true recipients of the promises of God. But this evidence is not merely subjective; it is not based on any virtues that we produce in and of ourselves. Rather, it is produced by the Holy Spirit, who is an objective presence in our lives.
This is one of many places where the Confession highlights the work of the Third Person of the Trinity. Although the Holy Spirit is not given a chapter unto himself, his presence is pervasive. Here the sanctified graces that he produces in us--together with his constant testimony that each of us is a true son or daughter of God--convince us that God will save us. 

This section closes by affirming the biblical metaphor (found in Ephesians 1 and elsewhere) that the Holy Spirit is God's down payment on eternity. The same Holy Spirit who lives in us today will transform us tomorrow--at the Second Coming of Jesus Christ. The Spirit's presence in our lives thus gives us strong confidence that one day we will inherit the new heavens and the new earth.

Dr. Philip Ryken is the president of Wheaton College and the author of numerous books, the most recent of which is Loving the Way Jesus Loves (Crossway, 2012).