This excerpt is taken from The Collected Writings of John Murray, Vol. 2: Systematic Theology, Chapter 23.
This progression (of sanctification) has respect, not only to the individual, but also to the church in its unity and solidarity as the body of Christ. In reality the growth of the individual does not take place except in the fellowship of the church as the fellowship of the Spirit. Believers have never existed as independent units. In God’s eternal counsel they were chosen in Christ (Ephesians 1:14); in the accomplishment of their redemption they were in Christ (2 Corinthians 5:14-15; Ephesians 1:17); in the application of redemption they are ushered into the fellowship of Christ (1 Corinthians 1:9). And sanctification itself is a process that moves to a consummation which will not be realized for the individual until the whole body of Christ is complete and presented in its totality faultless and without blemish. This points up the necessity of cultivating and promoting the sanctification of the whole body, and the practical implications for responsibility, privilege, and opportunity become apparent.
If the individual is indifferent to the sanctification of others, and does not seek to promote their growth in grace, love, faith, knowledge, obedience and holiness, this interferes with his own sanctification in at least two respects. 1) His lack of concern for others is itself a vice that gnaws at the root of spiritual growth. If we are not concerned with, or vigilant in respect of the fruit of the Spirit in others, then it is because we do not burn with holy zeal for the honor of Christ himself. All shortcoming and sin in us dishonors Christ, and a believer betrays the coldness of his love to Christ when he fails to bemoan the defects of those who are members of Christ’s body. 2) His indifference to the interests of others means the absence of the ministry which he should have afforded others. This absence results in the impoverishment of these others to the extent of his failure, and this impoverishment reacts upon himself, because these others are not able to minister to him to the full extent of the support, encouragement, instruction, edification and exhortation which they owe to him.
We see, therefore, the endless respects in which interaction and intercommunication within the fellowship of the saints are brought to bear upon the progressive sanctification of the people of God. “If one member suffers, all the other members suffer with it; and if one member is honored, all the others rejoice with it” (1 Corinthians 12:26). The truth of our inter-dependence within the solidarity of the body of Christ exposes the peril and contradiction of exclusive absorption in our own individual sanctification. How eloquent of the virtue which is the antonym of independence and aloofness are the words of the apostle: “And he (Christ) gave some apostles, and some prophets, and some evangelists, and some pastors and teachers, for the perfecting of the saints unto the work of the ministry, unto the edifying of the body of Christ; until we all come in the unity of the faith and of the knowledge of the Son of God, unto a perfect man, unto the measure of the stature of the fulness of Christ”! (Ephesians 4:11-13; Romans 12:4ff.; 1 Corinthians 12:12ff.; Colossians 2:19).
By what do theologians mean when they refer to the doctrine of union with Christ? Simply put, union with Christ refers to the language of “in Christ,” which is common throughout the New Testament. Kevin DeYoung rightly notes that this sort of language is found 216 times in the New Testament. In regards to the relationship between salvation and Union with Christ, John Murray says this, “Union with Christ is really the central truth of the whole doctrine of salvation…Indeed the whole process of salvation has its origin in one phase of union with Christ and salvation has in view the realization of other phases of Union with Christ.” Murray here has in mind the language of the Apostle in his letter to the Ephesians, where Paul says that God the father has blessed us “in Christ, with every spiritual blessing in the heavenly places” (Eph 1:3). 
The Reformers too understood that our salvation – all spiritual blessings – are only found within our union with Christ. John Calvin says it this way, “…that so long as we are without Christ and separated from him, nothing which he suffered and did for the salvation of the human race is of the least benefit to us. To communicate to us the blessings which he received from the Father, he must become ours and dwell in us. Accordingly…we are said to be engrafted into him and clothed with him.” Citing Romans 6:5, Calvin rightly understands the direct connection between the salvation of the human race and the blessings which Christ received from the Father. Indeed, we have been “united with him in a death like his” and “united with him in a resurrection like his” (Rom 6:5). Therefore, we must understand that the totality of spiritual blessings which we receive are our salvation, and can only be found in our union with Christ in his death and resurrection. Herman Bavinck rightly sums up this concept, “All the benefits of grace therefore lie prepared and ready for the church in the person of Christ…Atonement, forgiveness, justification, the mystical union, sanctification, glorification, and so on – they do not come into being after and as a result of faith but are objectively, actively present in Christ.”
 Richard Gaffin notes that the primary language for union with Christ is found in the “in Christ/the Lord” language, with other variations such as “with,” “for us” and “for our sins.” By Faith and Not By Sight, 41
 Kevin DeYoung, A Hole in our Holiness, 95
 John Murray, Redemption Accomplished and Applied, 161
 In another of his writings, Murray notes that it would be “exegetically impossible” to separate the scope of spiritual blessings from those blessing mentioned in the immediate succeeding context – adoption (vs. 5), redemption and forgiveness of sins (vs. 7), the knowledge of the mystery of God’s will (vs. 9) the inheritance (vs. 11), and the seal of the Holy Spirit (vss. 13, 14). Murray, Collected Writings Vol. 2, 126
 In his book By Faith and Not By Sight, Richard Gaffin cites Murray as particularly helpful in understanding Union with Christ. Gaffin summarizes Murray by saying “To sum up: present union with Christ – sharing with him in all he has accomplished and now is, by virtue of his death and resurrection – is, as much as anything, at the center of Paul’s soteriology. Page 45
 John Calvin, Institutes of the Christian Religion 3.1.1, 349
 Herman Bavinck, Sin and Salvation in Christ, 523. In agreement with Bavinck, Kevin DeYoung says this, “Union with Christ is not a single specific blessing we receive in our salvation. Rather, it is the best phrase to describe all the blessings of salvation, whether in eternity past (election), in history (redemption), in the present (effectual calling, justification, and sanctification), or in the future (glorification).” A Hole in Our Holiness, 94.