occurrence of the word. Yet union is a word that today litters Christian essays, articles, sermons, and books. I use the verb 'litters' not to belittle the proper use of the doctrine, but to highlight what I think is often its overuse as the supposed overarching meta-soteriological doctrine explaining salvation in the New Testament.
To me, it seems safest and most helpful to stick with bibilical language as much as possible. "Our union with Christ" or speaking of any particular blessing as coming to us "through union with Christ" aren't incorrect concepts per se. It's just that they are often used so broadly and freely that they are open to misunderstanding especially when not nailed down. As is often asked - what "union" are we talking about? Federal, legal, mystical...? What does one mean by "through union with Christ?"
I think it's similar in some ways to those who would insist that the doctrine of election be front and center when the gospel is preached. Only the elect 'hear' the gospel with ears of faith, yet they don't necessarily need to know up front how and why that is true in order to trust in Christ for forgiveness of sins. That said, the gospel doesn't claim that God sent Jesus to die for everyone and thus make it theoretically possible for everyone to be saved if only everyone would believe. Christ came for the elect. "All that the Father gives me will come to me, and whoever comes to me I will never cast out (John 6:37). Again, the biblical language should be our guide. See examples of gospel preaching in Acts. Is divine election present in the gospel? Indeed, it is the ground, yet most often in the background.
I think it's similar in some ways to those who would insist that the doctrine of election be front and center when the gospel is preached. Only the elect 'hear' the gospel with ears of faith, yet they don't necessarily need to know up front how and why that is true in order to trust in Christ for forgiveness of sins. That said, the gospel doesn't claim that God sent Jesus to die for everyone and thus make it theoretically possible for everyone to be saved if only everyone would believe. Christ came for the elect. "All that the Father gives me will come to me, and whoever comes to me I will never cast out (John 6:37). Again, the biblical language should be our guide. See examples of gospel preaching in Acts. Is divine election present in the gospel? Indeed, it is the ground, yet most often in the background.
The doctrine of our union with Christ is central in salvation. As John Calvin wrote, "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" (Institutes 3.1.1). Yet as a doctrine it remains most often in the background not the foreground when Christ crucified is preached. In a word, faith isn't directly engaged by preaching "our union with Christ", especially when it isn't unpacked. That is how I'm presently thinking about this.
Unpacking "union with Christ" - from R. Scott Clark at Heidelblog:
Definitions
There is also apparently some confusion about what is meant by “union with Christ.” This is understandable because the doctrine has three or four aspects and, in contemporary discussion, all participants have not always been as cautious as necessary to make sure we are talking about the same aspect at the same time in the same way.
Louis Berkhof (1873–1957) represented the mainstream of the Reformed tradition when he spoke of the “federal union” that all the elect have with Christ (Systematic Theology, 448). This aspect of union is relative to the eternal, pre-temporal (before time) “covenant of redemption” (pactum salutis) between the Father and the Son (and the Holy Spirit). According to Ps 110, John 17 and other passages, the Father gave to his Son a people and the Son volunteered to be their Mediator, their federal representative, and their Savior; i.e., to earn their salvation. This is one of the three or four aspects of our union with Christ. For more on this see the chapter on the “Covenant Before the Covenants” in Covenant, Justification, and Pastoral Ministry.
Berkhof wrote of a second aspect of our union with Christ, which he called the “union of life” (ibid). This union refers to the natural, organic relation that all humans have with the first Adam, who was the federal representative of all humanity (Rom 5). The corollary to our natural union with Adam, in whom we would have entered in glorious life had he (and we in him) obeyed the commandment of life (“you shall not eat”). In the covenant of redemption God constituted a union between the Son, who would be the Last Adam (1Cor 15) and his people. Implicitly, the Holy Spirit was a party to this covenant as that person who would apply redemption to the people given to the Son. The Second Adam (Rom 5), Jesus, fulfilled that covenant of works for all those whom he represented, for whom he died and for whose justification he was raised.
We might also speak of a third aspect of our union with Christ, which we might call decretal union, i.e., the union that exists between Christ and his people by virtue of God’s decree to elect, in Christ, some out of the mass of fallen humanity to redemption. Paul spoke to this aspect of our union with Christ when he wrote that we were chosen “in Christ” before the foundations of the world (Eph 1). This aspect is, of course, a corollary to the federal union and the union of life mentioned above.
The last aspect is mystical union (or sometimes referred to as “existential union”) and it refers to the subjective application of redemption purposed from eternity in the decree, covenanted among the Trinitarian person in the pactum salutis, accomplished by Christ in his active and suffering obedience, and applied to the elect by the Holy Spirit. Mystical union is, as Berkhof put it, that “intimate, vital, and spiritual” connection “between Christ and his people, in virtue of which He is the source of their life and strength, of their blessedness and salvation” (Systematic Theology, 449).And also...
That faith which secures eternal life; which unites us to Christ as living members of his body; which makes us the sons of God; which interests us in all the benefits of redemption; which works by love, and is fruitful in good works; is founded, not on the external or the moral evidence of the truth, but on the testimony of the Spirit with an by the truth to the renewed soul (Systematic Theology, 3.68).
…The first effect of faith, according to the Scriptures is union with Christ. We are in him by faith. There is indeed a union between Christ and his people, founded on the covenant of redemption between the Father and the Son in the counsels of eternity. We are, therefore, said to be in Him before the foundation of the world.
…But it was also, as we learn from the Scriptures, included in the stipulations of that covenant, that his people, so far as adults are concerned, should not receive the saving benefits of that covenant until they were united to Him by a voluntary act of faith. They are ‘by nature the children of wrath, even as others.’ (Eph. ii.3) They remain in this state of condemnation until they believe. Their union is consummated by faith. To be in Christ, as to believe in Christ are, therefore , in the Scriptures, convertible forms of expression. They mean the same thing, and therefore, the same effects are attributed to faith as are attributed to union with Christ” (Ibid, 3.104)
So says Charles Hodge (1797–1878), who taught at Old Princeton for about fifty years, on the relation between faith and union.