I think Fuller was Piper's mentor at Fuller Seminary. (I don't know that that Fuller is related to the namesake of the seminary.) Fuller was opposed to dispensationalism, but his views on law and gospel maybe went too far in the other direction and came close to equating law and gospel, from what I understand. Dennis McFadden is our resident Fuller Seminary expert, so I'm sure he could provide more details.
Allegedly this relationship is the reason for some problematic remarks on justification that Piper has made through the years that seemed to contradict sola fide. I haven't read much of Piper, but my recollection is that the most problematic statements were in "Future Grace." Thus, some found it to be somewhat ironic that last decade Piper became a champion of imputed righteousness and took on N.T. Wright. Some pointed out to him that certain statements in "Future Grace" didn't seem to fit with this emphasis, so I understand in the latest version those statements were clarified. But then he wrote an intro to Dr. Schreiner's book on sola fide which
reignited the controversy for some. (Most of the ones that I saw get worked up about it are Clarkians, For what it's worth.) Schreiner himself caught some flak for a previous work entitled "The Race Set Before Us" but not quite as much as I would have expected.
Anyway, here is the statement in question:
Obviously this doesn't jibe with the "free grace" theology that came out of Dallas Seminary. Certainly it doesn't fly with the (allegedly) rationalistic followers of Gordon Clark, at least a couple of whom weighed in on the TGC comment thread, who tend to end up at the same place as the teaching of the early Zane Hodges. (Hodges' later teaching was actually much more problematic.) The question is, is Piper's statement compatible with what the old Calvinistic divines called free grace?
For more on Piper and Fuller (and Wilson) you can search the Heidelblog. (I had linked some pages but then messed up and broke the links.) For a defense of Piper's statement by Mark Jones, see
here. My guess is that you could line up quotes from Reformed luminaries who appear to agree with Piper, as Jones did, and line up quotes from some others that appear to disagree.
I think there is a Q&A on Piper's site about Fuller. I think he still does these. You can maybe read up on some of that and then send in a question. Who knows, maybe he will answer it in light of the complaints over his recent book introduction unless he already has.