The Catechism makes a rather heavy distinction between the "We believe" of the Nicene Creed and the "I believe" of the Apostles Creed (cf # 167). Why then did the new English translation of the Mass render it, "I believe?"
The new English translation of the Creed to “I believe” is to bring the text in conformity with the authorized Latin text which has “Credo,” namely, “I believe” for the opening word of the Creed.
You are correct in noting that the text of the Creed coming forth from the Council of Nicaea used the Greek word Πιστεύομεν (pisteuomen) a first‐person plural meaning “we believe.” It was worded in this way because it came forth as a statement of all the bishops, speaking as a body, or college of bishops. Thus they rightfully said “we believe.”
However in the years that followed, when the Creed was brought into the liturgy, the form was switched to the first person singular Πιστεύω (pisteuo) “I believe.” This change was made because the Creed was now said by the individual believer, in the context of the liturgy.
This liturgical adaptation of the text is quite ancient and has been respected by the Church ever since. As the Greek liturgy moved to Latin in the West, the first person singular, “Credo,” (I believe) was thus used.
Your frustration related to harmonizing the new translation with the Catechism is understandable. However, it is possible to expect too much of liturgical texts, which have a particular context and are, of their nature more brief, when it comes to expressing the faith. The Catechism however is able to develop richer aspects of the teachings of the faith, and to explore historical and/or biblical roots. Thus, rather than to see the Catechism and the Mass in competition or conflict, one can see them as complementary, in this case, the Catechism helping to further articulate what the liturgy announces.
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