The third part of the Heidelberg Catechism, which is all about good works, is called “Thankfulness.” Good works in Heidelberg 64 are called “fruits of thankfulness.” In question 86, the first question in the third section of the Heidelberg, the first reason given for why we should do good works is to show our thankfulness to God. One of the prooftexts in this section in 1 Corinthians 6:20, in which Paul exhorts us to good works because of the fact that we were bought with a price. Thankfulness for that fact, for our having been bought with a price, motivates those good works.
Romans 12:1 likewise exhorts us to good works as the sacrifice of our whole lives to God. That cannot be a propitiatory or expiating sacrifice, since Christ is that sacrifice. The sacrifice instead is a thank offering, which is confirmed by Paul calling it our “reasonable service”- that is, the service (latreia, meaning service as worship) that is reasonable, appropriate. Our salvation can elicit no other response than this dedication of the whole life to God.
In short, all these speak of good works as flowing out of thankfulness. But how can I be thankful for what I am not sure I have? In other words, how can I do good works if they are not done out of a motivation of thankfulness? Are they even good works if they are not done out of that motive?
I think they are not. Indeed, Heidelberg 91 tells us that good works are only those done for God’s glory. But if I do good works out of fear of wrath, or to gain something, am I doing them to God’s glory, or for my own ends? Good works must flow from thankfulness to God. To the extent any of our works are good at all, they must proceed from that motive.
But then, good works cannot be done at all without at least some assurance of salvation. For how can I be thankful for what I am not sure I have?
In Heidelberg 86, true, it goes on to tell us that we should do good works “so that we may be assured of our faith by the fruits thereof.” But this is not the primary, but a secondary reason, and is presented that way- “then, also”. It clearly is telling us that good works will strengthen that assurance. As we see the power of Christ begin to work in us, that gives us additional confirmation that His promise is true. But this cannot be the primary basis for our assurance since those good works are not even good works until they have the element of thankfulness in them, which assumes the presence of at least some level of assurance of salvation.
So if faith will always produce good works, then faith must always, even in its most basic form, include assurance of salvation. This is not to say the assurance will always be 100% firm, that the true believer will never waver or be plagued with doubt. But assurance cannot be something added to faith later, since without some element of assurance no sanctification would really be possible. Those that say that some level of sanctification is necessary before one can have assurance of faith therefore are mistaken, not understanding the nature of true faith, and not understanding the necessity of thankfulness to drive good works that have any element of real goodness in them.