"We repeat, then—a sense of utter helplessness is the first prerequisite to any sound conversion. There is no salvation for any soul until it looks away from itself, looks to something, yes, to Someone, outside of itself!"
I am thankful that God is willing to save us even when our grasp of the gospel may be partial or defective. None of us has a comprehensive or perfect grasp of it.
Piper is saying that one can reject the doctrine of Justification--that's what the imputation of Christ's righteousness to us is (and our's to Him on the cross as our substitution), and still have a gospel that saves! That is heretical!