I was looking up something else and accidentally clicked on a bookmark to an 1860s German-American Methodist hymnal that's archived on Hymnary.org. And I got to reading the list of first lines (a lot start with "Ach"!) in hopes of finding one I recognized... and sure enough, there was one in the Bs: "Brüder! wir sind nun versammelt." The database doesn't have much info beyond the handful of hymnals in which it appears and when each one was published, but it has scans from one of those hymnals with very clear text. I thought I knew what the tune should be, despite there not being any music printed in this little songbook, but I wasn't completely certain until I got to the last two lines of the first verse:
Darum Brüder, ernstlich betet,
So wird Manna uns beschert.
You might recognize it a little better in English:
Brethren, pray, and holy manna
Will be showered all around.
Yes, it's "Brethren, We Have Met to Worship"! I can't find any indication which is the original and which is the translation, though the German hymnals were all printed in the US after the publication of the English version; the German version has several verses that aren't in the English version at all. (Now I'm debating whether to offer my services to transcribe and link these hymns to their English counterparts... as if I need another job that doesn't pay....)
ETA: Looked at it a little more closely, since En said she was curious about the extra verses. Looks like there's only one verse, about apostates, that's completely without precedent in the hymn we know and that may come from an alternate version found in a Brethren in Christ hymnal; the German then skips the verse that's in some hymnals encouraging people to pray with any penitents who might be present, and then splits the "Let us love our God supremely" verse into three verses with suitable elaboration/embellishment. Some interesting shifts in emphasis, too--"brink of woe" becomes "brink of Hell," for example, and "tell them that He will be found" becomes "tell them that He's seeking them."
Also interesting and kind of cool: the section of the hymnal where this song appears is entitled "Wachen und Beten" (Watch and Pray). In the Baptist Hymnal, it's in the "Family of God" section!