Marie,
Long time no see!
Even though we are Congregationalists, we use the the Heidelberg for adult catechesis. Here are several factors that I find helpful.
First, the historical context. Jacobius Arminius was only 3 years old at the time the HC was written (1563). While he could have been starting to
think about the extent of the efficacy of the atonement, it is certain that the particular
twist he gave to the Gospel would not be widely circulated until several decades later. In 1563 "all mankind" would not have been the hot-button phrase it would become later when misused as a supposed prop for Arminianism.
Second, the context within the HC. Extent and efficacy are addressed in part prior to Q.37:
Q.20 Will all men, then, be saved through Christ as they became lost through Adam?
No. Only those who, by true faith, are incorporated into him and accept all his benefits
Q.30 Do those who seek their salvation and well-being from saints, by their own efforts, or by other means really believe in the only Savior Jesus?
No. Rather, by such actions they deny Jesus, the only Savior and Redeemer, even though they boast of belonging to him. It therefore follows that either Jesus is not a perfect Savior, or those who receive this Savior with true faith must possess in him all that is necessary for their salvation.
Third, the Heidelberg is neither primarily a systematic theology nor an evangelistic tool. Its primary audience is the believier; its language is pastoral; its first goal is to strengthen, comfort and disciple the new--and even old--Christian.
