If a Priest is present at Mass, does he have to concelebrate?

That’s correct. If the priest were reciting the parts of the priest, and most specifically the consecration, he would be concelebrating, not assisting in choir. Being in choir is precisely a way of being present at a liturgy as a cleric (thus maintaining the distinction from the laity) without exercising one’s function/office. Bishops can also celebrate in choir if they choose to have a priest celebrate the Mass (bishops don’t act as concelebrants for priests).

One distinction, if a bishop is present, the bishop must officiate the Mass, which means that the bishop gives the opening and final blessing.

Thanks for the qualification. I had forgotten that detail.

If we are getting into distinctions, is this correct if the Bishop is outside his diocese?

I believe so. At a recent wedding (I’m a wedding sacristan) the bishop of Fargo presided in choir within the Diocese of Ft. Wayne-South Bend. I gave him a mic so he could do the parts required of him (not that a mic is absolutely necessary, but we need to wire celebrants when there are video crews).

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