Answer

God’s commandments as recorded in Deuteronomy 5:6–21 and Exodus 20:2–17 seem to add up to more than ten, so it is not perfectly clear how their numbering is to be reckoned. The numbering used in your RCIA class is consistent with the traditional Catholic catechetical formula. Numbered this way, the commandment prohibiting idolatry is not left out but, rather, is considered to be a part of the first commandment.