based on this , thus, we can say this...

if you say, one is impossible to know whether he is saved or not,
I would argue:
based on ... we can firmly say YES, one can know.

however,

if somebody else insists on that once saved always saved no matter what we do,
I would argue:
based on ... we can firmly say 忍耐不到底的, 必然不得救, thus, what we do does matter indeed.

ONE is saved because of his faith-100%
ONE is saved because of his deeds-100%

all depends how we approach or from which perspective.