One of the most important theological subjects is that which deals with how we are to view good works in the life a believer. The Apostle Paul, everywhere, explains that good works do not, in any sense, play into our justification (i.e. our right standing) before God (Rom. 4:1-8; Eph. 2:8-9; Phil. 3:7-9; and Titus 3:4-7). However, the Scripture also speaks of believers having been "created in Christ Jesus for good works" (Eph. 2:10), that we are to be "zealous for good works" (Titus 2:14 and 3:1) and that Jesus commends the righteous for what they did for His sake (Matt. 25:31-36). So how do we reconcile the fact that we are not accepted on the basis of our good works and yet that God accepts our good works as good works done by us--when we know how imperfect and sin-tainted they are? In Institutes (3.17.8), John Calvin explained how a believer's good works are justified by the perfection of Jesus so that they are reckoned to him as their own good works before God. When he entered in on an explanation of the phrase, "imputed for righteousness" in Psalm 106:30-21, Calvin first suggested that we must be clear that we are speaking of "what place [good works] are to hold after justification by faith has been established." Up front, Calvin carefully distinguishes between the place of works before and after justification. Calvin understood that while the Scriptures speak of the alien righteousness (i.e. the righteousness of Christ) imputed to us for justification, there is a personal righteousness that followed which we receive by virtue of our union with Christ in sanctification. The good works lived out by the believer are made acceptable by the saving work of Christ. In this sense we can speak of a double justification: 1) the justification of the believer by faith alone in Christ alone apart from any works; and 2) the justification of the good works of the believer after justification. The good works of the believer who has been justified by faith alone in Christ alone are imputed to the believer as if they were his or her own. This is not to say that they, in any sense, play into the believer's justification before God. Justification is merely the forgiveness of sins and the imputed righteousness of Jesus alone to the one who believes in Him. Calvin goes to great lengths to make this clear when he says, "Justification, moreover, we thus define: The sinner being admitted into communion with Christ is, for his sake, reconciled to God; when purged by his blood he obtains the remission of sins, and clothed with righteousness, just as if it were his own, stands secure before the judgment-seat of heaven."  Concerning God's acceptance of our good works after justification Calvin suggested:

Forgiveness of sins being previously given, the good works which follow have a value different from their merit, because whatever is imperfect in them is covered by the perfection of Christ and all their blemishes and pollutions are wiped away by his purity, so as never to come under the cognizance of the divine tribunal...and the imperfection which is wont to sully even good works being buried, the good works which are done by believers are deemed righteous, or; which is the same thing, are imputed for righteousness.

In short, far from teaching justification by faith plus works, Calvin distinguished between "imputed righteousness" with regard to Abraham's justification before God (Gen. 15:6) and the "imputed righteousness" of Phinehas' act of obedience referenced in Psalm 106:30-31. Calvin explicated this in full when he wrote:

Paul finds nothing stronger to prove justification by faith than that which is written of Abraham, he “believed God, and it was counted unto him for righteousness,” (Rom. 4:3; Gal. 3:6). Therefore, when it is said that the achievement of Phinehas “was counted unto him for righteousness,” (Psalm 106:30, 31), we may argue that what Paul contends for respecting faith applies also to works. Our opponents, accordingly, as if the point were proved, set it down that though we are not justified without faith, it is not by faith only; that our justification is completed by works. Here I beseech believers, as they know that the true standard of righteousness must be derived from Scripture alone, to consider with me seriously and religiously, how Scripture can be fairly reconciled with that view. Paul, knowing that justification by faith was the refuge of those who wanted righteousness of their own, confidently infers, that all who are justified by faith are excluded from the righteousness of works. But as it is clear that this justification is common to all believers, he with equal confidence infers that no man is justified by works; nay, more, that justification is without any help from works. But it is one thing to determine what power works have in themselves, and another to determine what place they are to hold after justification by faith has been established. If a price is to be put upon works according to their own worth, we hold that they are unfit to appear in the presence of God: that man, accordingly, has no works in which he can glory before God, and that hence, deprived of all aid from works, he is justified by faith alone. Justification, moreover, we thus define: The sinner being admitted into communion with Christ is, for his sake, reconciled to God; when purged by his blood he obtains the remission of sins, and clothed with righteousness, just as if it were his own, stands secure before the judgment-seat of heaven. Forgiveness of sins being previously given, the good works which follow have a value different from their merit, because whatever is imperfect in them is covered by the perfection of Christ, and all their blemishes and pollutions are wiped away by his purity, so as never to come under the cognizance of the divine tribunal. The guilt of all transgressions, by which men are prevented from offering God an acceptable service, being thus effaced, and the imperfection which is wont to sully even good works being buried, the good works which are done by believers are deemed righteous, or; which is the same thing, are imputed for righteousness.1

If someone were to respond by asking "Why do we need good works imputed to us if we already have the imputed righteousness of Christ for our justification?" Westminster Confession of Faith ch. 15 sect. 5 & 6 brings this discussion within the context of rewards. Note carefully what how they distinguish--as Calvin did--between works with regard to justification and works with regard to rewards--as well as they specific use of the word "accepted" with regard to the good works of the believer:

5. We can not, by our best works, merit pardon of sin, or eternal life, at the hand of God, because of the great disproportion that is between them and the glory to come, and the infinite distance that is between us and God, whom by them we can neither profit, nor satisfy for the debt of our former sins; but when we have done all we can, we have done but our duty, and are unprofitable servants: and because, as they are good, they proceed from his Spirit; and as they are wrought by us, they are defiled and mixed with so much weakness and imperfection that they can not endure the severity of God's judgment.

6. Yet notwithstanding, the persons of believers being accepted through Christ, their good works also are accepted in him, not as though they were in this life wholly unblamable and unreprovable in God's sight; but that he, looking upon them in his Son, is pleased to accept and reward that which is sincere, although accompanied with many weaknesses and imperfections.

Additionally, Belgic Confession article 24, "Of Man's Sanctification," articulates the place of good works in the life of the believer in a similar manner:

It is impossible for this holy faith to be unfruitful in a human being, seeing that we do not speak of an empty faith but of what Scripture calls "faith working through love," which leads a man to do by himself the works that God has commanded in his Word. These works, proceeding from the good root of faith, are good and acceptable to God, since they are all sanctified by his grace. Yet they do not count toward our justification-- for by faith in Christ we are justified, even before we do good works. Otherwise they could not be good, any more than the fruit of a tree could be good if the tree is not good in the first place. So then, we do good works, but nor for merit-- for what would we merit? Rather, we are indebted to God for the good works we do, and not he to us, since it is he who "works in us both to will and do according to his good pleasure-- thus keeping in mind what is written: "When you have done all that is commanded you, then you shall say, 'We are unworthy servants; we have done what it was our duty to do.' " Yet we do not wish to deny that God rewards good works-- but it is by his grace that he crowns his gifts. Moreover, although we do good works we do not base our salvation on them; for we cannot do any work that is not defiled by our flesh and also worthy of punishment. And even if we could point to one, memory of a single sin is enough for God to reject that work. So we would always be in doubt, tossed back and forth without any certainty, and our poor consciences would be tormented constantly if they did not rest on the merit of the suffering and death of our Savior.