This is the 18th part of a series of posts adapted from a paper I presented at a New Covenant Theology think tank in upstate New York in July 2010.
If sanctification is the work of the Holy Spirit in us, why do believers — who have received the Spirit — still need instruction and exhortation?
First, it is important to remember that believers are still imperfect this side of glory. As we have seen, the incarnate Christ as God-Man was the prototype of the believer given the Holy Spirit. But unlike us, the incarnate Jesus’ communication with the Holy Spirit was perfect. In Christ, the Spirit’s communication was complete. Abraham Kuyper explains this relationship:
There are three differences between this communication of the Holy Spirit to the human nature of Jesus and that in us:
First, the Holy Spirit always meets with the resistance of evil in our hearts. Jesus’s heart was without sin and unrighteousness. Hence in His human nature the Holy Spirit met no resistance.
Secondly, the Holy Spirit’s operation, influence, support, and guidance in our human nature is always individual, i.e., in part, imperfect; in the human nature of Jesus it was central, perfect, leaving no void.
Thirdly, in our nature the Holy Spirit meets with an ego which in union with that nature opposes God; while the Person which He met in the human nature of Christ, partaking of the divine nature, was absolutely holy. For the Son having adopted the human nature in union with His Person, was cooperating with the Holy Spirit.
We as believers fail to cooperate fully with the Holy Spirit. Immature believers, or those with certain weaknesses or besetting sins, need further instruction in ethics to aid their cooperation with the Spirit of Christ.
Paul tells the church at Corinth that its members are immature. “ But I, brothers, could not address you as spiritual people, but as people of the flesh, as infants in Christ.  I fed you with milk, not solid food, for you were not ready for it. And even now you are not yet ready,  for you are still of the flesh” (1 Cor 3:1–3).
Paul tells the Romans that some of their brothers are weak: “As for the one who is weak in faith, welcome him, but not to quarrel over opinions” (Rom 14:1). Indeed Paul explains that he has been sent as an apostle to bring encouragement: “For this reason I write these things while I am away from you, that when I come I may not have to be severe in my use of the authority that the Lord has given me for building up and not for tearing down” (2 Cor 13:10).
Stephen Westerholm writes: “As long as believers remain ‘in the flesh,’ the risk of succumbing to temptation remains. And as T. J. Deidun notes, those external imperatives are to be seen chiefly as a sign of “imperfect liberation.”
Thomas Schreiner doesn’t root the need to provide exhortation or explanation of an ethic of love solely in the imperfection or immaturity of believers, but he does assert the need for it:
For Paul, love does not float free of ethical norms but rather is expressed by such norms. In some ways Paul’s ethic is rather general, for he does not give specific guidance for each situation. He realizes that in many situations wisdom is needed to determine the prudent and godly course of action (Eph. 5:10; Phil. 1:9–11;Col. 1:9–11). Paul does not have a casuistic ethic that prescribes the course of action for every conceivable situation, but neither does he simply appeal to the Spirit and freedom without describing how life in the Spirit expresses itself. The notion that Paul appeals to the Spirit for ethics without any ethical norms is contradicted by his parenesis. Nor should the Pauline theme of obedience be identified as legalism, for the new obedience is the work of the Spirit in those who are the new creation work of Christ. Nor does it diminish the work of the cross, for the cross is the basis and foundation for the transforming work of the Spirit in believers.
Our sanctification is achieved by a union with Christ through His Spirit.
Paul’s exhortations and exposition serve to encourage the cooperation of with the Holy Spirit in the believer.
Paul exhorts not by showing the believer’s shortcomings through a comparison to the law – an external code that engenders sin, and thus resistance to the Spirit – but by encouraging those a reliance on the Spirit that brings the fruit of the Spirit of Galatians 5:22–23.
Next: Completed by the Spirit Part 19: Imperatives Rooted in the Indicative
 Abraham Kuyper, The Work of the Holy Spirit (London: Funk and Wagnall’s, 1900), 101.
 Stephen Westerholm, Israel’s Law and the Church’s Faith: Paul and His Recent Interpreters (Grand Rapids, MI: Wm. B. Eerdmans, 1988), 214.
 T. J. Deidun, New Covenant Morality in Paul (Rome: Editrice Pontifico Istituto Biblico, 1981, 2006), 209.
 Thomas R. Schreiner, New Testament Theology: Magnifying God in Christ (Grand Rapids: Baker Academic, 2008), 656.