The Heidelberg Catechism is outlined or structured around three (3) points or sections, often referred to as Guilt (Q.3-11), Grace (Q.12-85), and Gratitude (Q.86-129). This outline (although not employing these exact terms) is made explicit in Q/A 2:
“Q.2. What must you know to live and die in the joy of this comfort?
A. Three things: first, how great my sin and misery are; second, how I am set free from all my sins and misery; third, how I am to thank God for such deliverance.”
The catechism is largely comprised of somewhat lengthy expositions of the The Apostles’ Creed (Q.22-58), The Ten Commandments (Q.92-115), and the Lord’s Prayer (Q.116-129). These things are commonly considered to be the ABC’s or building blocks of the Christian faith and life.
As you can see, most of the “gratitude” section of the catechism in centered around the ten commandments and the Lord’s prayer. And so how we live and pray is really about showing our gratitude to God for our salvation in Christ.
Q/A 86 marks the beginning of the “gratitude” section of the catechism. It says:
Q.86. Since we have been delivered from our misery by grace through Christ without any merit of our own, why then should we do good works? A. Because Christ, having redeemed us by his blood, is also renewing us by his Spirit into his image, so that with our whole lives we may show that we are thankful to God for his benefits, and that he may be praised through us, and further, so that we may be assured of our faith by its fruits, and by our godly living our neighbors may be won over to Christ.
Q/A 86 basically asks the age-old question, if we are really saved by grace alone, and not by works, then why should we as believers do good works? If our works do not merit anything (and they don’t!), then why does it matter how we live?
Paul anticipates a similar objection to the grace of God in the gospel in Romans 6:1, where he writes, “What shall we say then? Are we to continue in sin that grace may abound?” (ESV) And he answers by saying “By no means!” (v.2)
The answer both in Romans 6 as well as in the Heidelberg is basically that our salvation by God’s grace in Christ includes much more than justification (as vitally important as that is). It also includes the new birth and sanctification (God’s work in us), which involves “renewing us by his Spirit into his image.”
As to why it matters how we lives as believers, Q/A 86 gives us at least four (4) reasons or purposes for the work of God’s grace in sanctification in our lives:
- Gratitude (“so that with our whole lives we may show that we are thankful to God for his benefits”)
- Praise to God (“that he may be praised through us”)
- Assurance (“so that we may be assured of our faith by its fruits”)
- Evangelism (“by our godly living our neighbors may be won over to Christ”)
The first two of these are God-ward (gratitude & praise), the third is in some way for our own benefit (growth in assurance that our faith is, in fact, genuine), and the fourth is for the benefit of others (that they might be won to Christ). This list is not meant to be exhaustive, but it is certainly a good starting point in demonstrating the importance of good works in the life of a Christian.
Either way the primary motivation (though certainly not the only proper motivation) for living the Christian life of good works is gratitude for God’s grace in our salvation. This is the same logic that the Apostle Paul applies in Romans 12:1-2, where he writes,
 I appeal to you therefore, brothers, by the mercies of God, to present your bodies as a living sacrifice, holy and acceptable to God, which is your spiritual worship.  Do not be conformed to this world, but be transformed by the renewal of your mind, that by testing you may discern what is the will of God, what is good and acceptable and perfect. (ESV)
It is in light of the mercies of God toward us in Christ that we are to present our bodies as a living sacrifice to God. Our primary motive for being transformed by the renewal of our minds is gratitude for the mercies of God.