It seems that Heidelberg here is not providing a scripture reference for the latter part of the answer (from "therefore"). It certainly makes sense, and I am not sure any other answer would make sense, but what I would like to know is what is the origin of putting it that way is, and if (or rather, how) a biblical case for it can be constructed.
Q 11: Is not God then also merciful?
Answer: God is indeed merciful, but also just; therefore his justice requires, that sin which is committed against the most high majesty of God, be also punished with extreme, that is, with everlasting punishment of body and soul.
Q 11: Is not God then also merciful?
Answer: God is indeed merciful, but also just; therefore his justice requires, that sin which is committed against the most high majesty of God, be also punished with extreme, that is, with everlasting punishment of body and soul.