The following by John Owen, in his work entitled The Doctrine of Justification by Faith, has been cited as proof of his holding to the priority of mystical/existential union as over and against the priority of justification:
"The foundation of the imputation asserted is union. Hereof there are many grounds and causes, as has been declared; but that which we have immediate respect unto, as the foundation of this imputation, is that whereby the Lord Christ and believers do actually coalesce into one mystical person. This is by the Holy Spirit inhabiting in him as the head of the church in all fullness, and in all believers according to their measure, whereby they become members of his mystical body. That there is such a union between Christ and believers is the faith of the catholic church, and has been so in all ages. Those who seem in our days to deny it, or question it, either know not what they say, or their minds are influenced by their doctrine who deny the divine persons of the Son and of the Spirit. Upon supposition of this union, reason will grant the imputation pleaded for to be reasonable; at least, that there is such a peculiar ground for it as is not to be exemplified in any things natural or political among men."
It is sound to consider the mention of imputation as referring to justification, as Francis Turretin himself states:
"Thus the imputation of righteousness is the foundation and meritorious cause of justification" (Institutes of Elenctic Theology, II.16.4.5).
But a preceding passage has Owen clarifying as to what aspect of union with Christ he was actually referring to:
"The first spring or cause of this union, and of all the other causes of it, lies in that eternal compact that was between the Father and the Son concerning the recovery and salvation of fallen mankind. Herein, among other things, as the effects thereof, the assumption of our nature (the foundation of this union) was designed. The nature and terms of this compact, counsel, and agreement, I have declared elsewhere; and therefore must not here again insist upon it. But the relation between Christ and the church, proceeding from hence, and so being an effect of infinite wisdom, in the counsel of the Father and Son, to be made effectual by the Holy Spirit, must be distinguished from all other unions or relations whatever."
In other words, Owen was stating that justification and its attendant benefits are the Christian's by virtue of their antecedent decretal (pactum salutis) and federal (historia salutis) union with Christ. He was really not referring to mystical/existential union, which is predicated upon justification by faith.
In another place on the same work, Owen further prioritizes justification:
"The plain truth is, the apostle speaks not one word of the necessity of our sanctification, or regeneration, or renovation by the Holy Ghost, antecedently unto our justification; a supposition whereof contains the whole force of this argument. Indeed he assigns our regeneration, renovation, and justification, all the means of our salvation, all equally unto grace and mercy, in opposition unto any works of our own; which we shall afterwards make use of. Nor is there intimated by him any order of precedency or connection between the things that he mentions, but only between justification and adoption, justification having the priority in order of nature: 'That, being justified by his grace, we should be heirs according to the hope of eternal life.' All the things he mentions are inseparable. No man is regenerate or renewed by the Holy Ghost, but withal he is justified; — no man is justified, but withal he is renewed by the Holy Ghost. And they are all of them equally of sovereign grace in God, in opposition unto any works of righteousness that we have wrought. And we plead for the freedom of God’s grace in sanctification no less than in justification. But that it is necessary that we should be sanctified, that we may be justified before God, who justifies the ungodly, the apostle says not in this place, nor any thing to that purpose; neither yet, if he did so, would it at all prove that the signification of that expression 'to be justified,' is 'to be sanctified,' or to have inherent holiness and righteousness wrought in us: and these testimonies would not have been produced to prove it, wherein these things are so expressly distinguished, but that there are none to be found of more force or evidence."
Michael Horton adds regarding the relationship of union with Christ to justification:
"Union with Christ is not to be understood as a 'moment' in the application of salvation to believers. Rather, it is a way of speaking about the way in which believers share in Christ in eternity (by election), in past history (by redemption), in the present (by effectual calling, justification, and sanctification), and in the future (by glorification). Nevertheless, our subjective inclusion in Christ occurs when the Spirit calls us effectually to Christ and gives us the faith to cling to him for all of his riches...Establishing the legal basis of this new relationship, union with Christ is first of all forensic...Taking root in the forensic soil of justification, from which it derives its effective power as well as its legal basis, union with Christ produces the life of Christ within believers, which bears the fruit of righteousness." (The Christian Faith, 587, 597)