The only truly useful knowledge is that which teaches us to seek the good and avoid the bad.
Literal
Only useful knowledge [definition-とは], good things [object-を] search-and-seek, bad things [object-を] avoid method [object-を] us [target-に] teach knowledge is.
とは frames a definition: 'as for what X is...' The sentence provides the definition via a complex relative clause modifying the final 知識. われわれ is the formal 'we.' 捜し求める ('to search for and seek out') is a compound verb intensifying the search. Philosophical and essayistic.