Paul's forcefully clear focus on Christ's substitutionary mediation in Romans 5 relates to justification. While there is room for a fuller presentation of Paul's doctrine of justification or a fuller discussion of Romans 5 (e.g., Paul's presentation of the law, imputation, transgression, Adam in 5:13-14), I concentrated here on clear and necessary conclusions from Romans 5: the righteousness resulting in divine approval at the last day comes to us as a free gift of the righteousness of Christ as the Second Adam and our mediator. It is his obedience to the covenant stipulations of the law imputed to us that forms the only ground of our justification, an eschatological verdict rendered now in Christ. The soteriology offered by Paul's opponents insofar as it is evidenced in Romans (and Galatians)--whether one sees it as Sander's "covenant nomism" or as any other kind of synthesis that somehow imports our works of obedience into our justification--is just what Paul's teaching in Romans 5 decisivwely undercuts. Any synthesis makes Christ's substitutionary life and death gratuitous and undermines God's grace.
Justification is indeed based on a human obedience to the law of God, and that loving obedience was entire and perfect in every respect, but no human after Adam--being helpless, impious enemies of God (cf. Rom 39-20) who have defaced the divine image are therefor devoid of the glory of God (3:23)--did or even could ever fulfill God's holy law for righteousness, except one: the one man, Jesus Christ. That is Paul's incontestable message in Romans 5 and has been a continuing hallmark of Reformed interpretation to this day.[Covenant, Justification, and Pastoral Ministry, The New Perspective, Mediation, and Justification, pp. 162-163]
Westminster Larger Catechism-Question 70: What is justification?
Answer: Justification is an act of God's free grace unto sinners, in which he pardons all their sins, accepts and accounts their persons righteous in his sight; not for any thing wrought in them, or done by them, but only for the perfect obedience and full satisfaction of Christ, by God imputed to them, and received by faith alone.
Heidelberg Catechism-Question 62: But why cannot our good works be the whole, or part of our righteousness before God?
Answer: Because, that the righteousness, which can be approved of before the tribunal of God, must be absolutely perfect, and in all respects conformable to the divine law; and also, that our best works in this life are all imperfect and defiled with sin.
Heidelberg Catechism-Question 63: What! do not our good works merit, which yet God will reward in this and in a future life?
Answer: This reward is not of merit, but of grace.