When we speak of good works being necessary for final salvation we need to make use of words and phrases to explain the meaning of this aspect of our salvation. By far the most common phrase I have come across in Reformation and Post-Reformation dogmatics concerning good works being necessary for final salvation is the language of “means and way.”
Thus, Robert Rollock is fairly representative in his choice of words:
I grant that the works of regeneration are necessary unto eternal life promised in the gospel, but not as merits or meritorious causes, but as the means and way wherein we are to proceed on from justification and regeneration unto glory and life eternal. They may also be said to be causes, after a sort, for they please God in Christ, and in some respects move him, but not as merits, but as effects of the only merit of Jesus Christ, whereof they testify.” (Works, 1:45)
If we consider a text such as Romans 8:13 we can get a better understanding of how this plays out,
“For if you live according to the flesh you will die, but if by the Spirit you put to death the deeds of the body, you will live.”
To put it crassly, we might say: if you do not mortify your sins you will not be saved. I fail to see how we can come to any other view based on a plain reading of the text. Mortification is a condition of “living.” Yet, this is where the value of Protestant scholasticism can help us, especially as we make distinctions between different types of conditions.
In Romans 8:13 we can understand the “if” in terms of the “uncertainty of the event.” According to John Owen, “this takes place where the condition is absolutely necessary unto the issue, and depends not itself on any determinate cause known to him to whom it is prescribed. So we say, ‘If we live, we will do such a thing.’ This cannot be the intendment of the conditional expression in this place. Of the persons to whom these words are spoken, it is said, verse 1 of the same chapter, ‘There is no condemnation to them.’”
Thus the conditional “if” in Romans 8:13 is not a type of condition whereby there is uncertainty for the true child of God. That does not, however, make the duty needless or unnecessary. Rather, the conditional “if” may be understood, says Owen, in terms of
“The certainty of the coherence and connection that is between the things spoken of; as we say to a sick man, ‘If you will take such a potion, or use such a remedy, you will be well.’ The thing we solely intend to express is the certainty of the connection that is between the potion or remedy and health. And this is the use of it here. The certain connection that is between the mortifying of the deeds of the body and living is intimated in this conditional particle.”
Now, like Rollock, Owen does speak of “cause and effect” as well as “way and means”. But by “cause and effect” we must not understand the matter “properly and strictly”, but in terms of “means and end”: “God has appointed this means for attaining that end…Means, though necessary, have a fair subordination to an end of free promise. A gift, and procuring cause in him to whom it is given, are inconsistent. The intendment, then, of this proposition as conditional is, that there is a certain infallible connection and coherence between true mortification and eternal life: if you use this means, you shall obtain that end; if you do mortify, you shall live. And herein lies the main motive unto and enforcement of the duty prescribed” (Works, 6:6).
Thus we can speak of mortification of our sinful nature as necessary for salvation while making the correct distinctions to ensure that the grace of God is not jettisoned in any way.
Good works are not, therefore, “merely evidence of sanctity and nothing more.” They are the “way and means” that God has ordained for his children to walk to glory. If we do not walk on this path we will not be saved. But, as Owen notes above, we shall walk on this path. In this sense, and this sense alone, we may say with the vast majority of Reformed theologians, good works are necessary for salvation.
Now let us consider 2 Thessalonians 2:13,
But we ought always to give thanks to God for you, brothers beloved by the Lord, because God chose you as the firstfruits to be saved, through sanctification by the Spirit and belief in the truth.  To this he called you through our gospel, so that you may obtain the glory of our Lord Jesus Christ.
How do we obtain the glory of our Lord Jesus Christ? If someone, not just John Piper, says we are saved through “that fruit and that faith” they may easily mean: “sanctification by the Spirit and belief in the truth” are the means and way (i.e., “through”) that God has appointed for believers to walk on the way to glorification.
So not only must we reckon with the distinction between the “right” and “possession” of life, and various types of conditions (so Owen), but we must also make use of the “means and way” language to explain what we mean by good works being necessary for final salvation. Francis Turretin sums this all up well,
“For since good works have the relation of the means to the end (Jn. 3:5, 16; Mt. 5:8); of the ‘way’ to the goal (Eph. 2:10; Phil 3:14); of the ‘sowing’ to the harvest (Gal. 6:7,8)…of labor to the reward (Mt. 20:1); of the ‘contest’ to the crown (2 Tim. 2:5; 4:8), everyone sees that there is the highest and an indispensable necessity of good works for obtaining glory. It is so great that it cannot be reached without them (Heb. 12:14; Rev. 21:27).”
This does not sound like good works are merely evidence of sanctity and nothing more as some have suggested!
If we believe that good works are either merely evidence of justification or they must be meritorious for justification then it makes sense why some would be upset with Piper’s language in the original offending article. I can understand that for those who do not have a deep acquaintance with Reformed scholastic theology. But the Scriptures and the Reformed tradition do not conceive of matters in quite this way. There is a real necessity (or conditionality properly understood) for salvation in non-meritorious terms that allows us to speak of “indispensable necessity of good works for obtaining glory.” I believe the language of “way and means”, understood in the broader context of “right and possession” is the best way to do justice to the Scriptures. If there is a better way and means of evaluating the biblical data then so be it; but I don’t believe “merely evidence” quite captures the force of passages like Romans 8:13.