Patrick Ramsey has a short piece on union with Christ at Meet the Puritans. Some brief comments:
It is possible to speak of a union between Christ and the elect in terms of the decree and the federal headship of Christ. But these senses are quite different from an actual or mystical union and are not under our purview.
This refers to the standard three-fold distinction of union with Christ referring to three points in time: eternity, the cross, and conversion. The problem, however, is that we are not federally united with Christ until conversion. Prior to that we are federally “in Adam” and you can’t be both “in Adam” and “in Christ” at the same time. Separating “actual” union from federal union is a tremendous, but very common mistake. Owen does a good job of explaining that Christ and the elect are united at the cross only in the plan and intention of God (via the Covenant of Redemption). Christ does not become our federal head/representative/surety until we enter the Covenant of Grace.
An actual union with Christ refers to the moment when a sinner is united to Christ at his conversion, or in the words of the Westminster Larger Catechism, in his effectual calling (Q&A 66-67).
This is another very important point. In Redemption Accomplished and Applied, John Murray states
It is calling that is represented in Scripture as that act of God by which we are actually united to Christ (cf. 1 Cor. 1:9)… [S]alvation in actual possession takes its start from an efficacious summons on the part of God and that this summons, since it is God’s summons, carries in its bosom all of the operative efficacy by which it is made effective. It is calling and not regeneration that possesses that character.
In the effectual call we are federally united to Christ and all the benefits he earned become ours because the effectual call is God making the Covenant of Grace (New Covenant) with us. In his commentary on Hebrews 8:6-13, John Owen explains
[I]n the description of the covenant here annexed, there is no mention of any condition on the part of man, of any terms of obedience prescribed unto him, but the whole consists in free, gratuitous promises…
It is evident that there can be no condition previously required, unto our entering into or participation of the benefits of this covenant, antecedent unto the making of it with us.
However, many argue that union with Christ is established through faith, after the effectual call. This leads to a significant logical dilemma: where does faith come from if it does not come from our union with Christ? Owen explains
(1.) God’s reckoning Christ, in our present sense, is the imputing of Christ unto ungodly, unbelieving sinners for whom he died, so far as to account him theirs, and to bestow faith and grace upon them for his sake.
This, then, I say, at the accomplishment of the appointed time, the Lord reckons, and accounts, and makes out his Son Christ, to such and such sinners, and for his sake gives them faith, etc. Exercising of love actually, in the bestowing of grace upon any particular soul, in a distinguishing manner, for Christ’s sake, doth suppose this accounting of Christ to be his; and from thence he is so indeed, — which is the present thesis.
And, — (2.) This may be proved; for, — [1.] Why doth the Lord bestow faith on Peter, not on Judas? Because Christ dying for Peter, and purchasing for him the grace of the covenant, he had a right unto it, and God according to his promise bestowed it; with Judas, it was not so. But then, why doth the Lord bestow faith on Peter at the fortieth year of his age, and not before or after? Because then the term was expired which, upon the purchase, was by the counsel of God’s will prefixed to the giving in the beginning of the thing purchased unto him.
What, then, doth the Lord do when he thus bestoweth faith on him? For Christ’s sake, — his death procuring the gift, not moving the will of the giver, — he creates faith in him by the way and means suited to such a work, Ephesians 1:18,19, 2:1, etc. If, then, this be done for Christ’s sake, then is Christ made ours before we believe. Else, why is faith given him at this instant for Christ’s sake, and not to another, for whom also he died? That it is done then, is because the appointed time is come; that it is done then for Christ, is because Christ is first given to him. I cannot conceive how any thing should be made out to me for Christ, and Christ himself not be given to me
Union must precede faith. Ramsey attempts a nuanced remedy.
The Catechism’s definition of effectual calling is broad, however, and it includes both the regenerating work of the Holy Spirit and the sinner’s personal act of faith. It thus allows for one to place union in connection with regeneration or with faith or with both. Since union involves a reciprocal relationship or what Reynolds called a mutual act, wherein “Christ exhibiteth himself unto us, and we adhere and dwell in him,” it is theologically legitimate to do all three.
This is fine if we first understand all that has been argued above and then understand faith’s role in terms of covenant “restipulation.” All covenants require a response of some kind. Nehemiah Coxe notes
If the Covenant be of Works, the Restipulation must be, by doing the things required in it, even by fulfilling its condition in a perfect obedience to the Law of it…But if it be a Covenant of free and soveraign Grace, the Restipulation required, is an humble receiving, or hearty believing of those gratuitous Promises on which the Covenant is established. (9)
So union with Christ can be said to be through faith insofar as reception through faith is our restipulation to the New Covenant (Covenant of Grace). But it must be understood that the union is established by God prior to our restipulation. Owen concludes
Let it be granted on the one hand, that we cannot have an actual participation of the relative grace of this covenant in adoption and justification, without faith or believing; and on the other, that this faith is wrought in us, given unto us, bestowed upon us, by that grace of the covenant which depends on no condition in us as unto its discriminating administration, and I shall not concern myself what men will call it.
[T]he covenant of grace does not first arise as a result of the order of salvation but precedes it and is its foundation and starting point. While it is true that the believer first, by faith, becomes aware that he or she belongs to the covenant of grace and to the number of the elect, the epistemological ground is distinct from the ontological ground.
In the second place, therefore, regeneration, faith, and conversion are not preparations that occur apart from Christ and the covenant of grace nor conditions that a person has to meet in toto or in part in his or her own strength to be incorporated in that covenant. Rather, they are benefits that already flow from the covenant of grace, the mystical union, the granting of Christ’s person. The Holy Spirit, who is the author of these benefits, was acquired by Christ for his own. Hence the imputation of Christ [here I would say “our legal New Covenant union with Christ] precedes the gift of the Spirit, and regeneration, faith, and conversion do not first lead us to Christ but are taken from Christ by the Holy Spirit and imparted to his own.
Bavinck, Reformed Dogmatics: Sin and Salvation in Christ, p. 524
Union with Christ is the New Covenant, which God makes with the elect in the effectual call. All redemptive blessings earned by Christ flow to the elect sinner through the New Covenant as the backbone of the ordo salutis.
For a much more detailed discussion, see New Covenant Union as Mystical Union in Owen.