Q: I was a visitor at a parish during the Christmas season and was stunned when the priest began singing “O Come Let Us Adore Him” during the elevation of both the host and chalice. Is this a permissible adaptation to the Roman Missal?
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A: It was trendy about 40 years ago to sing that refrain as a memorial acclamation at Christmas Time. But it was never one of the official options and is not now.
There are two other difficulties with the practice you describe. The priest is not supposed to participate in the memorial acclamation. He introduces it, but he remains silent as the people sing.
Second, one reason we lost “Christ has died” as an acclamation is that it is an acclamation about Christ instead of an acclamation to Christ. After the consecration, the community greets Christ on the altar, just as they acclaim his presence at the conclusion of the gospel: “Praise to you, Lord Jesus Christ.” The refrain to the Christmas carol is sung about Christ, not addressed to him.