Hi C
Piper is a Calvinist.I am not though. He is also a tireless defender of justification by faith.The "New Perspective On Paul" is a movement that says when Paul spoke of justification he didn't mean the classical definition of it, but that Paul felt it pertained to Jew an Gentile being reconciled in Covenant. Piper and others feel this chips away at the real meaning of justification. He has debated with NT Wright on this. Wright is a major spokesman for NPP. Piper wrote a book about all this also.
As a Catholic,I am sure my friend Chesterton,would be glad to know,we are NOT justified by works.