Michael Pahl has an interesting post concering this question. I have struggled with this. Lest you think I deny justification, let me clarify that I do not. I affirm that we are justified by the imputation of the righteousness of Christ and that this justification is completely outside of us.
I still struggle with this question: is justification the heart of the gospel? Do we really want to say that justification=gospel? I think Michael provides some very important questions to think about in regard to this issue.