All people that on earth do dwell,
sing to the Lord with cheerful voice.
Him serve with mirth, his praise forth tell;
come ye before him and rejoice.
According to Bob Mills, who wrote a chapter in the book Does God LOL?, the word "mirth" in Old One Hundredth was originally "fear". It was changed to "mirth" in the Scottish Psalter to be more in keeping with the words of Psalm 100, on which this song is based.
Makes sense to me! And so does this admonition from St. Théophane Vénard who suffered terribly before he was brutally martyred for his faith:
Be merry, really merry.
The life of a true Christian should be a perpetual jubilee,
a prelude to the festivals of eternity.
Here, here! Let the joy begin -- and never end! I'm in! How about you?