Thanks for the link. I believe you have seriously misunderstood Piper on this. From the link you shared:
But is Romans 2:13b a hypothetical statement? When Paul says, "Not the hearers of the Law are just before God, but the doers of the Law will be justified," does he really mean: They would be justified if there were any, but there aren't any "doers of the law." Or to put it another way, Does "doers of the Law" refer to sinless, perfect law-keepers? Could Paul call a person a "doer of the law" who sins, but who loves God and loves the law and hates his own sins and confesses them and casts himself on the mercy of God revealed in the law itself?
I think he could. And I think he does. So I believe verse 13 means: Not the hearers of the law are just before God, but the ones who will be acquitted at the last judgment will be those who 1) love God's law, and 2) depend on his help to live according to the truth that they have, and 3) trust God for his mercy when they stumble.
Hearing the Gospel Is Essential for Salvation
But now listen carefully, lest you misunderstand: Apart from the preaching of the Gospel, and the awakening work of the Holy Spirit that leads to faith in Christ, nobody is saved today* in this way. That's Paul's point in these first two-and-a-half chapters of Romans. The reason for this - the reason no one is saved in this way, apart from hearing the gospel of Christ - is that everyone without Christ "suppresses the truth in unrighteousness" (Romans 1:18). Everyone hardens his heart against repentance (Romans 2:5).
In other words, you might ask me (as some of you have), "Theoretically, could people be saved today who haven't heard of Christ, if they were "doers of the law" the way you described (namely, people who 1) love God's law, and 2) depend on God's help to live according to the truth that they have, and 3) trust God for his mercy when they stumble)? And I would answer, "Yes, theoretically they would be saved" (and God would cover their sins by the blood of Christ, similar to the way he did for the saints in the Old Testament), but it never happens. The reason we need aggressive, loving evangelization among all the peoples of the world is because people everywhere suppress the truth and will not yield to God without the sovereign work of the Holy Spirit in their lives (Romans 1:18), and this Spirit works savingly only through the Gospel of Christ crucified and risen. God has shut all men up in disobedience (Romans 11:32) so that his Son, Jesus Christ, will be the conscious Object of faith among all the peoples (see Acts 4:12; Romans 10:13-15). (See further support in John Piper, Let the Nations Be Glad [Grand Rapids: Baker Book House, 1993], pp. 131-166.)
http://www.desiringgod.org/resource-library/sermons/there-is-no-partiality-with-god-part-2