A: The 1563 answer to that famous conundrum, "it all depends on what the meaning of is is."
Well, perhaps not exactly. The Heidelberg Catechism developed in the midst of the splintering of the early Protestant movement between its Lutheran, Zwinglian and Calvinist branches. While the breakup of the Protestant movement can be traced along geopolitical lines in the devolution of the Holy Roman Empire, it also divided theologically. Heidelberg reflects that split, particularly over the meaning of the sacrament of communion.
What did Jesus mean when he said, at the Last Supper, "this is my body"? Luther interpreted the words literally and thus argued that Christ is literally present in the bread of communion. Zwingli understood it metaphorically arguing that Jesus could not have meant this bread is literally my body when he was, in fact, bodily present in the room. Calvin took a third tack, arguing that Christ is spiritually present in the elements of communion. That classic Reformed perspective is reflected in the liturgy of our time when the officiant says "set aside these common elements for a sacred purpose."
On such distinctions churches split and empires rise or fall.
In addition to articulating a Reformed sacramental theology, Heidelberg stresses the essential Reformed commitment to stewardship as a way of living. We are saved, the catechism stresses, in order to serve the purposes of God in the world. As such, how we use the resources at our disposal is a reflection of God at work in our lives.
Here's a taste of the catechism, which opens with this question and response:
Q. 1. What is your only comfort, in life and in death?
A. That I belong--body and soul, in life and in death--not to myself but to my faithful Savior, Jesus Christ, who at the cost of his own blood has fully paid for all my sins and has completely freed me from the dominion of the devil; that he protects me so well that without the will of my Father in heaven not a hair can fall from my head; indeed, that everything must fit his purpose for my salvation. Therefore, by his Holy Spirit, he also assures me of eternal life, and makes me wholeheartedly willing and ready from now on to live for him.