And unless you quickly prevent them may be expressed as “If you don’t do something immediately [or, quickly].”

They will do still greater things does not necessarily mean they will do more impressive things, but simply more, as Good News Bible expresses it with “they will do even more.” Goldstein has “they will accomplish even more.”

And you will not be able to stop them should not be difficult to translate.

Quoted with permission from Bullard, Roger A. and Hatton, Howard A. A Handbook on 1-2 Maccabees. (UBS Helps for Translators). New York: UBS, 2011. For this and other handbooks for translators see here.