I am reading stellar whitepaper and understood:

In fact, since ill-behaved nodes contribute nothing to safety, no protocol can guarantee safety without the well-behaved nodes enjoying quorum intersection on their own. Two quorums overlapping only at ill-behaved nodes will again be able to operate like two different FBAS.

And got stuck on the following:

In short, FBAS can survive Byzantine failure by a set of nodes B ⊆ V iff enjoys quorum intersection after deleting the nodes in B from V and from all slices in Q.

It will be helpful if someone can provide an overview of the Stellar consensus protocol explaining above sentence.