The phrase "그대들은 어떻게 살 것인가" can be translated as "How will you all live?" or "How will you live?" addressing a group.
- 그대들 (geudaedeul): you (plural, poetic or literary tone)
- 은 (eun): topic particle
- 어떻게 (eotteoke): how
- 살 (sal): to live (stem form)
- 것인가 (geosinga): a question form meaning "will (do something)?"
So the whole sentence is asking how the addressed people will live their lives.
