I recant my original position. Excommunication liturgy may be used in church, although it must not be done so as to disrupt or replace the normal course of the worship service. I still don't think Mat. 16:18-19 refers to excommunication. The Heidelberg Cathechism was written fairly early during the Reformation, when many great theologians of the 17th century where yet to come and add further insights to the understanding of Scriptures. That is why I prefer the Westminster shorter/longer cathechism over the HC.

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Jean-David Jutras
URCNA
Alberta