O mira circa nos tuæ pietatis dignatio!
O inæstimabilis dilectio caritatis: . . .
O marvelous dignation [(deeming-worthy)]
of [this] dutifulness of yours that encompasses us!
O dilection [(love)] of inestimable charity: . . .
2010 Roman missal:
O wonder of your humble care for us!
O love, O charity beyond all telling, . . .
1979 Book of common prayer:
How wonderful and beyond our knowing, O God, is your mercy and loving-kindness to us, . . .
Ex(s)ultet. Pietas is the love and duty (dutiful conduct, even fealty) owed to a superior. Hence the condescension ("humble care") of the most recent translation of the Roman missal.
For the Ex(s)ultet in an early 20th-century edition of the Missale Gothicum (c. 700), go here (there are later critical editions, but they are not online):
O mira circa mos tuae pietatis dignacio
O instimabilis dileccio caritatis
But the Ex(s)ultet itself may well be earlier still.